Adafruit Industries Adafruit Industries Diffused 5mm Slow Fade Flashing RGB LED - 10 Pack
These are very interesting 5mm diffused RGB LEDs - instead of having 4 pins to control 3 LEDs, they have only two leads - power and ground. When powered, the LEDs perform a slow fade through the rainbow, from red to orange to yellow, etc till they get back to red. See the video below for the timing and look. There is no way to change the 'program' or rate, its burned into a little chip that is inside the LED itself. They do diffuse nicely so you can the color changing from any angle. The forward voltage of the whole LED is about 3.4VDC but you can drive them from a lithium coin cell like a CR2032 and they'll just be a little dimmer.
Adafruit Industries Diffused 5mm Slow Fade Flashing RGB LED - 10 Pack
Usually ships in 5-7 business days.