What happens when you shine blue light on an orange object?

Petr Macháně asks:
"What happens if there is orange object [seen in] blue light? What color should I paint part of the object that's in the light? Blue and orange are opposites on color wheel. I am just looking for advice on how to think about this."

Petr, the color you need to mix is the apparent color you see. The word "apparent" is important here, because a given color must be seen in context. Those context cues tell you what color of light seems to be shining in the scene. If the light is white, the color you see will match the color of the object, but in colored light, everything changes.

In the diagram above, for example, we see see a checkerboard with a multicolored bar hovering over it. In the left sub-rectangle, it appears to be lit by yellow light because our visual system is tricked by the yellow shift of all the colors inside the rectangle. On the right we believe it's lit by blue light. The actual color note of the intersection piece is the same on both sides: It's a neutral gray.

So the color you paint is the color of the light plus the local color of the object. Sometimes this results in a color canceled out to neutral gray. But this effect can enhance a color, too. If you put green light on a green surface the result will be even brighter green. You see this in folds of flesh, where the reflected light shines back into a fold of skin and makes it a brighter orange." 

"To answer your question, if you put an orange object in blue light, the color you mix may be a rather dull gray, but if you do it right, it will look orange. Try it and see if it works for you."

Previously: Color Constancy