I did this a couple (and i only man a couple) of years ago when i had a fiat bravo. I agree with the spray choices given by Markos but i used a laquer on mine after and got a really decent finish. Best bet is to get a scrap light strip from a an ebay breaker and have a play. Just make sure you use plenty of light coats. You could even spray the inside if you can get the unit apart. Then if it's anygood swap it with the one on your car to see how visable the lights shine through. Job done then.
Right, lets get one thing out of the way to start with. Anything you do to the lens will make it illegal. Full stop, please don't try and argue, I've had enough with the aftermarket HID issue.
Now, if you were to absent mindedly forget to read my above statement
, then what you would need would be a tinted lacquer that is formulated to adhere to acrylic (PMMA). Ideally you want a 'neutral density' tint, ie one that attenuates all wavelengths equally. That way you do not change the actual colour of the light being transmitted through the lens.
It is entirely possible that you could apply a coat of such a lacquer, and depending on the level of tint, still have a lamp that complied with the legally required light outputs (still doesn't make it legal tho, but hey-ho).
Under no circumstances should you apply this to the inside of the lens. Part of the lens contains the Rear Reflex Reflectors (you know, the bits that light up when the headlights of the car behind hit them). These work on the principle of total internal reflection to reflect the light from the headlamps back in the direction it came, and require the lens surface to be perfectly clean. Any contamination on the surfaces of these prisms will stop them working, and applying a paint/ lacquer would stop the whole thing dead in it's tracks. Now you may be thinking 'so what, it's only a little glowything...I'll do without it' but what you need to remember is that when your car is parked up at the side of an unlit road, that 'little glowy thing' is the only thing that will let other cars know it is there when it's dark. In fact their sole purpose in life is to protect your car from being rear ended whilst it is sleeping peacefully at the side of the road.
Please don't mess with the reflexes, your bella may never forgive you
P.S. Some reflexes will have a cover weded over the back of them to prevent dust etc killing the reflexes. If this is the case with the GTV lamp then you could try lacquering the inside, however paints/ lacquers usually don't deal with the heat very well and quickly degrade and burn off, also the lamp will still appear red as the ambient light will travel through the red of the lens before reflecting off the inner surface (without travelling through the lacquer) and back out of the lens. The only colour that it has travelled through is the red of the original lens material. Doing it this way, the only effect you are having is on the light emitted by the bulbs. Best apply any paint/lacquer to the outer surface, that way you will get the effect you are after.