You should be able to adapt the code on this page

Interactive Color Calculator

to your needs. Just check out the HTML and Javascript sourcecode - it is free to use, just make sure you acknowledge its use. You can download the Javascript code here

Javascript Code

The calculator allows color selection using the HSL, CMYK or RGB models which may not be relevant to your needs so take out what you don't want. I would suggest using HSL with a clickable colorwheel display to get your client started with a raw color to refine to the exact hue, saturation etc as they desire. If you need any help understanding how it all works, just PM me.

Hope this helps.