I attached a PDF of Bill Bowdens fading circuit. It explains how it works, and things you can change. You will need 6 op amps, if you want to control each RGB color individually, or to reduce part count and board size you can cross-fade two colors. I used 220 uf capacitors for a much longer fade.
It doesn't take long to breadboard, parts aren't too critical. So you can use what's on hand or experiment. You only need one voltage divider, and can be shared with the other 1 or 2 colors. Adding a pot to control the rate isn't needed, component tolerances will give you a slightly different frequency for every triangle generator, but might be nice if you want to make one color dominate.