There are basically two approaches to this. They differ by where the mapping is done:

You can use an image map, and handle the coordinate-to-RGB translation on the server side. This approach gives you a lot of flexibility with our graphics, at the expense of making the coordinate mapping more difficult the fancier you get.

Alternatively, you do the "translation" on the client side, by doing something like filling a table with colors, and adding a link to each cell that embeds the RGB code in the URL. Such a table is easy to generate from Perl.