Ok, I've already asked this in #banana but my girlfriend wasn't happy about the answers (sorry guys).

Here is the problem:

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For example, when a red light strikes an object a red shadow is formed.

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Originally posted by wiki
[edit] Colored shadows
If white light is produced by separate colored light sources, the shadows are colored.

Illuminate a room with a red light, and the shadows are exclusively gray, or dark. Illuminate the shadows with a white light, and the shadows are green. Where both lights are blocked, or in other words where the shadows intersect, the shadows are gray. Away from the intersection, where the red light is blocked the shadows are green, and where the white light is blocked the shadows are red. In other words, light colors shadows or brightens them, according to the complementary color of the light blocked to cast the shadow. In the case of white and red lights, the complement of white is red; with white and green lights, the complement of white is green.

In the absence of multiple light sources, colored lights illuminate spaces where other lights are not blocked. In the above example, the red shadow cast by blocking white light is not a shadow with the white light off, but it is illuminated in red.

In the absence of white light, colored lights blocked by an opaque surface cast shadows in the colors complementary to the lights blocked. For green light, red shadows, and vice-versa; blue, orange; yellow, purple; intermediate light, intermediate shadows

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Light diffracts round objects, and diffracts ("bends") more for different colours (due to their wavelength). It's like when you've got the lunar eclipse, and the moon appears red. Red light has a larger (~650nm) wavelength than blue (~475nm) and can diffract more.

So, the reason if you shine a red light at something and its shadow being red is because the red light diffracts slightly round the object. If you put the object very close to the surface the shadow is being projected onto you'll notice that only the edges are red.