Just stumbled across a neat LED that shows rainbowcolors by itself:
http://www.superbrightleds.com/moreinfo/component-leds/rgb-slow-color-changing-led/778/
Has only 2 pins and with a few of these you can make a real multicolor rainbow. Just switch them on separately with a four seconds delay.
it would work but they don't have accurate timing in their internal processors, so the temporal drift would make them do random things as they heated and cooled differently. It only takes a miniscule change at 'megahertz' to multiply quickly into a real distortion.
Makes sense...
Would be a bit overkill to use these LEDs and syncronize them by sampling their color. 
o_lampe:
Would be a bit overkill to use these LEDs and syncronize them by sampling their color.
Not only overkill but it would not work. The only control you have over them is on or off, you have no way to synchronize them.
It only takes a miniscule change at 'megahertz' to multiply quickly into a real distortion.
That by itself can be an interesting effect.
Or you can synchronize them by turning them off / on quickly periodically.
Would be a bit overkill to use these LEDs and syncronize them by sampling their color.
You can sample its current: at some point it is going to cycle all of leds off - that's when you can synchronize them. It does require two pins per led for this to work.