dirty green or yellow linear Alps board, starting at $35 shipped

[jacobolus]

This thing needs a good clean, but the price is right. Among common linear switches that can still be easily used in modern keyboards, complicated linear Alps (this should be green Alps, or possibly yellow) are quite nice:
http://www.ebay.com/itm/390805824898

The keycaps are nice double-shot ABS, which will be some nice shades of gray if retrobrited. (At the moment though they look kind of olive colored.)

[False_Dmitry_II]

Sure that it's not just dirty? On the off chance I did get it, I don't think I'd actually try to change the color of the keys.

[jacobolus]

Not sure what you mean. It’s definitely substantially yellowed: the original plastic colors of the case and keycaps are gray, gray, and gray, all very colorless/neutral, whereas the current colors are olive, olive, and sort of beige color. :-)

If you like the olive color though, then just cleaning the keycaps would be fine. If you want gray (which IMO looks very nice), then you may want to put it through a retrobrite process.