Eugene, see if the manufacturer gives the range of color wavelengths for the 3 watt RGB LED. The small blue LEDs I used (at 470nm) weren't, well, blue enough. The RGB LEDs I used also didn't go far enough into the spectrum to work well.
Sounds pretty neat refitting a old color enlarger. You'll lose a lot of light intensity bouncing around in a dichroic mixing box--this is a complete guess, but I'd say you'll need several of the big RGB LEDs in the mixing chamber to match the original halogen bulb.
ic-racer, yes, I deliberately made the grid bigger than the 4x5 frame as I knew the corners wouldn't be as bright as the middle. The grid I built is six inches square illuminated, and it's cropped in slightly from that by the bottom portion of the box/housing that holds it all.