I have a red object, but hit by blue light it looks blue. why?

What color would you expect it to look? Purple? Just think of an example from the real life - does yellow light shining off a blue car make the car green? AFAIK it does not. In other words - run a Google on colors and color theory. OpenGL, true, is a discrete system bound by discrete boundaries (it can in no instance take into account all of the existing factors that you come across in nature) - hence it has to settle with a lesser form of it - a mathematical formula. These (formulae) are another thing you might study.

Crispy

uhh... thats weird. a red object only reflects red light, and a blue light doesnt emit any red light, so the red object should appear to be black...

ahem, I meant it should look black.

Lighting is additive, unlike paint. If you mix two light sources together, you get a lighter color, not a darker one. At least thats what they always taught us in school.

But I don''t have two light sources! :-/

for example, I have a red CUBE hit by blue LIGHT. what I get is a blue cube.
I hope it''s clear now ..