public void paint(Graphics g) {

    // your class colors... by the way,  this is not java
    // convention to lower case class names.  Should be Colors
    colors colorChooser = new colors();
  
    Color c = colorChooser.decide(A[i])
    g.setColor(c);
    // Color is now set :-)

    // now you can g.drawRect(....) with the correct color.
}