The color of an LED determines the minimum voltage needed for it to emit light. This has to do with the colors of light having different amounts of energy. A colored LED is not a white LED with a colored piece of plastic around it. It actually emits only light of a given color.

Red LEDs require about 1.5 – 1.8 V
Green LEDs require about 2.3 V
Blue LEDs require about 2.9 – 3.1 V

Where can I get replacement LEDs to use with the KidWind SimpleGEN?
Where can I get the electronic components used in Vernier physics experiments?