I used a semi-transparent acryl paint (spray). From the outside, they look black, but from the inside, you nearly can´t see a difference. So, if the lights are on, they are as bright as a serial light would be.
Mondeo 1993 GLX 1,8 16 V 115hp 7,5 x 16 Wheels with 215/40-16 Black Taillights custom grille Supersprint muffler Stroboscope lights in headlights and grille