Please see the code

import java.awt.*;        // Defines basic classes for GUI programming.
     import java.awt.event.*;  // Defines classes for working with events.
     import java.applet.*;     // Defines the applet class.
     
     public class ColoredHelloWorldApplet
                       extends Applet implements ActionListener {
                          
           // Defines a subclass of Applet.  The "implements ActionListener"
           // part says that objects of type ColoredHelloApplet are
           // capable of listening for ActionEvents.  This is necessary
           // if the applet is to respond to events from the button.
                             
        Font textFont;    // The font in which the message is displayed.
                          // A font object represent a certain size and
                          // style of text drawn on the screen.
        TextField T1;
        TextField T2;
        TextField T3;
        int redColor, greenColor, blueColor;
        String as, ag, ab;
        Button bttn;
        public void init() {
     
               // This routine is called by the system to initialize 
               // the applet.  It sets up the font and initial colors
               // the applet.  It adds a button to the applet for 
               // changing the message color.
               
            setBackground(Color.lightGray);
                  // The applet is filled with the background color before
                  // the paint method is called.  The button and the message
                  // in this applet will appear on a light gray background.
     
            redColor=0;
            greenColor=0;
            blueColor=0;
            
            
            textFont = new Font("Serif",Font.BOLD,24);
                  // Create a font object representing a big, bold font.
     
     /*
     		TextField T1=new TextField("",12);
     		TextField T2=new TextField("",12);
     		TextField T3=new TextField("",12);
     		*/
     		
     		T1=new TextField("",12);
     	 	T2=new TextField("",12);
     	 	T3=new TextField("",12);
     		
     		
     		add(T1);
     		add(T2);
     		add(T3);
            bttn = new Button("Change Color");
                  // Create a new button.  "ChangeColor" is the text
                  // displayed on the button.

            bttn.addActionListener(this);  
                  // Set up bttn to send an "action event" to this applet
                  // when the user clicks the button.  The parameter, this,
                  // is a name for the applet object that we are creating.
                    
            add(bttn);  // Add the button to the applet, so that it 
                        // will appear on the screen.
     
        }  // end init()
        
        
        public void paint(Graphics g) {
              
              // This routine is called by the system whenever the content
              // of the applet needs to be drawn or redrawn.  It displays 
              // the message "Hello World" in the proper color and font.
     
           Color mixColor=new Color(redColor, greenColor,blueColor);
           //Color mixColor=new Color(255,255,50);

           //g.setColor(mixColor);
           g.setColor(mixColor);
           //g.setColor(new Color(redColor, greenColor, blueColor));
           g.setFont(textFont);       // Set the font.
           
           g.drawString("This Color changes", 20,70);    // Draw the message.
           //T1.setText("Is this function reached");
     
        }  // end paint()
        
     
        public void actionPerformed(ActionEvent evt) {
        
              // This routine is called by the system when the user clicks
              // on the button.  The response is to change the colorNum
              // which determines the color of the message, and to call
              // repaint() to see that the applet is redrawn with the
              // new color.
            // T1.setText("23");
            
            if (evt.getSource() == bttn)
            {
              
              as=T1.getText();
              ag=T2.getText();
              ab=T3.getText();
       	  	  as=as.trim();
          	  ag=ag.trim();
              ab=ab.trim();
          
           redColor= Integer.parseInt(as);
           greenColor= Integer.parseInt(ag);
           blueColor= Integer.parseInt(ab);
           
           repaint();  // Tell system that this applet needs to be redrawn
          }
        }  // end init()
        
     } // end class ColoredHelloWorldApplet

I tested everything
redColor, greenColor,blueColor are getting the values
The only problem with teh code is in these lines

Color mixColor=new Color(redColor, greenColor,blueColor);
//Color mixColor=new Color(255,255,50);

//g.setColor(mixColor);
g.setColor(mixColor);

Can anyone help please?

When I call g.setColor(Color.red) it works but the above code
Why? :(

I got it to work
Can someone please help me how do I change the background rather than changing just text color?

setBackground(Color c);

Thanks, it worked.
Only the last problem
Whenever someone enters value except betwen 0 and 255 throw an error message

I spent few hours but not able to understand why this doesnt works!

if((redColor>255 || redColor<0)||(greenColor>255 || greenColor<0)||(blueColor>255 || blueColor<0))
{

String title = "Greetings"; // Shown in title bar of dialog box.
String message = "Please enter values only between 0 and 225.";
JOptionPane.showMessageDialog(null, message, title,
JOptionPane.INFORMATION_MESSAGE);

// g2d.setColor(Color.red);
// g2d.drawString("Please enter values only between 0 and 225", 20,70);
}
else
{
g2d.setColor(new Color(redColor, greenColor,blueColor));
}

import java.awt.*;        // Defines basic classes for GUI programming.
     import java.awt.event.*;  // Defines classes for working with events.
     import java.applet.*;     // Defines the applet class.
     import java.util.StringTokenizer;
     import javax.swing.*;
          
     public class ColoredHelloWorldApplet
                       extends Applet implements ActionListener {
                          
           // Defines a subclass of Applet.  The "implements ActionListener"
           // part says that objects of type ColoredHelloApplet are
           // capable of listening for ActionEvents.  This is necessary
           // if the applet is to respond to events from the button.
                             
        Font textFont;    // The font in which the message is displayed.
                          // A font object represent a certain size and
                          // style of text drawn on the screen.
                          
        TextField T1;
        TextField T2;
        TextField T3;
        int redColor, greenColor, blueColor;
        int increment;
        /*String as, ag, ab; */
        //Button bttn;
        public void init() {
     
               // This routine is called by the system to initialize 
               // the applet.  It sets up the font and initial colors
               // the applet.  It adds a button to the applet for 
               // changing the message color.
               
            setBackground(Color.lightGray);
                  // The applet is filled with the background color before
                  // the paint method is called.  The button and the message
                  // in this applet will appear on a light gray background.
     
            redColor=10;
            greenColor=20;
            blueColor=30;
            increment=1;
                      
            textFont = new Font("Serif",Font.BOLD,24);
                  // Create a font object representing a big, bold font.
          		
     		T1=new TextField("",12);
     	 	T2=new TextField("",12);
     	 	T3=new TextField("",12);
			//String message = "Line1\nLine2";
    		//JOptionPane.showMessageDialog(this, message);
     		
     		
     		add(T1);
     		add(T2);
     		add(T3);
            Button bttn = new Button("Change Color");
                  // Create a new button.  "ChangeColor" is the text
                  // displayed on the button.

            bttn.addActionListener(this);  
                  // Set up bttn to send an "action event" to this applet
                  // when the user clicks the button.  The parameter, this,
                  // is a name for the applet object that we are creating.
                    
            add(bttn);  // Add the button to the applet, so that it 
                        // will appear on the screen.
     
        }  // end init()
        
        
        public void paint(Graphics g) {
              
              // This routine is called by the system whenever the content
              // of the applet needs to be drawn or redrawn.  It displays 
              // the message "Hello World" in the proper color and font.
              
           Graphics2D g2d = (Graphics2D)g;
                            
           Color mixColor=new Color(redColor, greenColor,blueColor);
     
           setBackground(new Color(redColor, greenColor, blueColor));
     
           if((redColor>255 || redColor<0)||(greenColor>255 || greenColor<0)||(blueColor>255 || blueColor<0))
           {

			 String title = "Greetings";  // Shown in title bar of dialog box.
	         String message = "Please enter values only between 0 and 225.";
        	 JOptionPane.showMessageDialog(null, message, title,
                                        JOptionPane.INFORMATION_MESSAGE);

    //		g2d.setColor(Color.red);
    //		g2d.drawString("Please enter values only between 0 and 225", 20,70);
           }
           else
           {
           	g2d.setColor(new Color(redColor, greenColor,blueColor));
           }
           
           increment=increment+1;
     
        }  // end paint()
        
     
        public void actionPerformed(ActionEvent evt) {
        
              // This routine is called by the system when the user clicks
              // on the button.  The response is to change the colorNum
              // which determines the color of the message, and to call
              // repaint() to see that the applet is redrawn with the
              // new color.
            // T1.setText("23");
                         
                         
                         String as1=new String();
                         String ag1=new String();
                         String ab1=new String();
                         
                         
                         
                         String as=new String();
                         String ag=new String();
                         String ab=new String();
                         
              as1=T1.getText();
              ag1=T2.getText();
              ab1=T3.getText();
              
       	  	  as=as1.trim();
          	  ag=ag1.trim();
              ab=ab1.trim();
          
           redColor= Integer.parseInt(as);
           greenColor= Integer.parseInt(ag);
           blueColor= Integer.parseInt(ab);
           
           repaint();  // Tell system that this applet needs to be redrawn
          
        }  // end init()
        
     } // end class ColoredHelloWorldApplet
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