Additive Light

Here we can see in these examples how different mixtures of red, green and blue light combine to make different colours.  You can can experiment with this in programs like Photoshop by adjusting the RBG colour sliders to see what different mixtures will look like.  Somewhat even mixtures of red, green and blue will result in greyish colours.  For completely grey colours, make sure the red, green and blue sliders are equal to each other.

The way light interacts with surfaces can be a little bit confusing, but hopefully the diagram below adds some clarity.  When we see something with our eyes that has the appearance of a certain colour, what we are seeing is the wavelengths (visible colours) of light that are not being absorbed by that surface, in other words, when we see something as yellow for example, what we are seeing is yellow light being reflected off a surface. 

A surface can only reflect the light that reaches it.  So if the light source is pure white with full amounts of red, green and blue wavelengths, we will see the true reflective property of the surface.  However, in many cases the light reaching an object’s surface will not have full amounts of red, green and blue.