Here's one I discovered by accident, get a permanent marker pen, in whatever colour you need. Run it around the bottom edge of the canopy, that is, the bit that sticks to the frame/fuselage. Leave it a while to dry, then stick it on with clearfix or white glue, something non solvent. It gives a subtle shade to the clear plastic & looks quite good, I used it on a 1/24 P 51 that I converted to a two seater pylon racer, it has a great greenish tint, rather like car windows. Also, check out the helicopter forum, this is a recent topic there