It may not be optimally coloured for photosynthesis, but it has plenty of chlorophyll and quite enough light gets through, as you can tell by holding up a leaf to the light.

It is summer time. Go and get a tan (if you can mange it in English sunshine!  :-)  )

You will get the idea!

Cheers,

Jon

Two factors are at work here.

Chlorophylls absorb best in the red and blue wavelengths, so basically 'waste' green light. In the case of copper beeches there are additional pigments that absorb well in the green range, and can pass the trapped energy on to the photosynthetic pathway, thus making them more efficient. 

Second, in general, the darker the color of the leaf, the more it is absorbing light, so a black leaf would theoretically be most efficient.  So, the plants that are more at risk are those with very pale pigmentation which means they are trapping a lot less of the light that a copper beech.