If your LEDs are red , the red tail light lens will intensify your color and will look more predominant than using a clear lens. Just food for thought.
Sort of... that's part illusion and a part missing. . .
If you have a clear LED rated at 100 millicandles, and a red LED rated at 100 millicandles, then the clear one will be attenuated by the lens filtering out the rest of the spectrum besides red. So, placed side by side, the red will look brighter compared to the clear. Also, a lens will disperse the red light better since that light is not impeded by the lens within itself ( in its material ) and so it will look larger than the LED's appear without a lens, ( given that the LED's are far enough away from the lens to develop a beam upon its rear ).
But on the other hand, if you have a clear LED that is rated at 10 candles and set behind a red lens and you hold it up to those others, the others will appear to be dark by comparison since 10 candles is 10,000 times brighter in the entire spectrum.
Last but not least... the old Thomas Edison trick of putting a mirror behind a light-bulb to catch and reflect its rearward rays of light, won't work with LED's since they are typically built to cast their light only in a very small conic section down their axial forward.
So just like light-bulbs, LED's are all different ratings, power consumption, and brightness depending on the model.
This reply might sound technical/picky, but... for more information that *is* actually more technical/picky and dispels a few myths... see here:
Test Your High-Power LED IQ
Hope this helps