spray paint (in a rattle can) for polycarbonate r/c car bodies; in "smoke" or similar color. tamiya paints are pretty good for this, but there are plenty of others too. the more you spray, the darker the canopy gets. wash the canopy first, rinse thoroughly & dry, then spray the inside of the canopy 'til you reach your desired tint.

there's also the dye method where you soak the canopy in a hot water & "rit" dye combination, but i've never found out where to get hold of said dye.

good luck.