dhenry:
Because of different forward voltage drops for those diodes (red leds tend to have the lowest forward drop), your set-up will not allow you to turn on blue / green leds when the red led is on.You should consider using 3 separate resistors for those leds if you intend to turn some of them on at the same time.
Thanks for the answers. I'm using only one led (RGB with for pins) and it have one cathode, thats why there is only one resistor. The blinking appears only with the Green and Blue led as you say, so that could be the problem!. I also used the For instead of the pot and the blinking stays there. This was just a practice for fun, now that I'm know that the problem is not the code but the LED, I could go on with other projects 
Thanks for your time!