White LEDS?

how come the white leds I've seen on the forums and other people's sites look "Yellow" in the off mode? from the front?

White LEDs use (usually) a blue led with phosphors.
The phosphors emit a yellowish light that mixes with the blue, resulting in a psuedo white. That's also why a lot of the luxeons seem to be a greenish color.

Sorry, I didn't really answer your question.

Those phosphors are yellowish in color.
I'm not sure if this is a reaction with the visible light or just that they are.

When a phosphor absorbs light of a particular color it will re-emit the light (fluoresce) at a 'lower frequency' color (where blue is high and red is low). Now remember that blue light from the LED die is absorbed by the phosphor and re-emitted as 'white' light. So what happens if it absorbs cyan? It will emit a more yellowish light. Yellow light is re-emitted as an orangish light and so on. Since daylight is a combination of all the above the phosphor will appear to be yellow or greenish-yellow.

That is about the best I can do to explain it. Can someone please try to say it better than me?

Cheers.

Another way to say it is that the phosphor absorbs blue and ultra-violet light in order to floresce in the visual (white) range. Thus, it is very good at absorbing blue. When turned off, it will reflect the ambient light, but absorb the blue part and reflect the rest. Take the white room light, remove the blue, and what's left is ... yellow.