Before doing the "tint" I'd check your local speed shops to see how strict the inspectors are in "your neck of the woods." NJ members have been failed on tinted taillights before, but enforcement can be a hit or miss thing. Consider leaving the reverse light area stock.

As a DIY experiment, there are some spray tints widely used . . . Nightshades, which is rather expensive per can but is available in smoke or red; some tint sprays from Japan; and the small cans of smoke tint from Pactra/Testors that R/C hobby folks use to tint their models windows or canopies.