At CopySense, I found a nifty little online utility by which one can easily compare two colors in the RGB color space. One can even increment or decrement the hex values of each color. Besides that, there’s a number of other useful utilities available at CopySense.
Btw, Martin Latter, who runs CopySense, does some really fascinating graphics work (see the following article at Computer Arts to learn more). He basically uses the open source program StructureSynth, some additional open source apps, and Sunflow, an open source global illumination renderer.