Actually, the popular circuit for this is Bill Bowden's 'Fading Red Eyes' circuit. It's been post on this board, easy to find on the web. The circuit is a low frequency triangle wave, using half of a LM324 quad opamp. Just duplicate it three times for red, green, and blue. Only need one voltage divider for a reference for all three. I used 470uf capacitors instead of the 22's because I want to see a wider range of colors. Also works better with 3 red LEDs, instead of two. The series resistors are fine.
Oh, the RGB LEDs with the 4 pins are junk (least the ones I got), the brightness isn't well matched for each color.
Natural, uncolored plastic makes for a great difuser (small disposiable cups). Home Depot has some nice frosted glass globes for light fixtures under $5 for the 6" diameter...