Originally Posted by DBall

The easiest way to do it is to open up the pic in photoshop or GIMP and find the color you want to preserve. These instructions are for PS, Gimp is pretty much the same, however.

Select it with the magic wand (and if you uncheck the "contiguous" box, it will select everything of that color)... you can fine tune what you select by using the lasso tool and the shift button to add to your selections or the alt button to subtract from it if you grab too much... zooming in is very helpful in this process.

Anyway, once you select everything of the color you want, right click and choose "select inverse" and then go to Image --> Adjustments --> Desaturate.

That's how to do it. If this isn't clear enough, I can do a little pictoral tutorial for it... it's damned easy.