[John Chapman (OP)]

Leaves are green because they reflect back green light. In other words, they absorb all other colours of light, particularly red. So if you could get inside a leaf shouldn't it be red or purple?

Or... If water reflects blue light why doesn't everything under water seem yellow?

Come to that, any transparent material that reflects one primary colour should appear to be the remaining colours if viewed from the opposite side to the light source. So why is a stained glass window the same colours when viewed from either side. And where do the remaining colours, and the light energy they contain, go?

[Madidus_Scientia]

It doesn't absorb green light. So the green light travels through the leaf, or stained window, as well as some being reflected.

[John Chapman (OP)]

Thanks Madidus_Scientia

So if something transparent absorbs one colour of light where does that colour go? What happens to the energy lost in that colour? Does red glass become warmer to conpensate for the absorbed energy in the blue and green light? Or does it take on a small amount of mass?

Also, consider the red brake lights on a car which are only red because white light is shone through a red lens. If only a very small bandwidth of light is allowed through, and half of that is reflected back (because the lens would appear red if viewed from the same side as the bulb) then you might expect that only 10%, say, of the light might pass through. But brake lights are virtually as bright as an uncovered bulb. Is the lens somehow changing the wavelengths of other colours of light to red so that more light passes through?

[lyner]

A simple filter, like you find on stop lamps, is just an absorber of other frequencies of light.
You can get much fancier filters which use interference due to thin layers of dielectric (transparent) material on a glass sheet. They are called Dichroic filters. They will reflect certain wavelengths and let others pass through. A set of them can be used, one after the other, to split a beam of light into different wavebands in order to analyse its spectral content. It is better than using absorptive filters because energy is not lost, so the system is more sensitive.
They sell 'dichroic' spotlights which look sparkly and attractive because you get bright colours with high intensity.
Aamof, insects (butterflies) and some flowers produce their vivid colours this way, too. Just one simple chemical (chitin, I think) can be used for all the colours instead of a whole set of different chemicals which are needed to produce pigments.

[Madidus_Scientia]

Yes, light that is not reflected will be absorbed as heat.