The translite is a single layer print over a thin diffuser panel. Translites don't have nearly as much colour depth that a standard screened backglass does which is why they always look " milky" when put next to a traditional backglass.
The diffuser sheet on the translate is very thin and was made to suit a certain type of light.. in this case.. the amount of light given off by the traditional incandescent bulb. Along come LEDs and are able to give off more light for less power consumption.. that's what makes them efficient.. energy in compared to light output.
But now you have created  several problems..

LEDs are uni directional not multi and so focus the beam to a very narrow width compared to traditional bulbs.
 As mentioned before, the translite was designed for a certain amount of light to be behind.. you have just multiplied that amount quite considerably and so will " wash Out" the colours.
The distance between the translite and LED is not large enough for the light beams from the LEDs to cross over and create an area of light as opposed to lots of beams.
The translite is not thick enough to diffuse the LED light internally.
Quite simply... the translite is not designed to have LED behind it... so it begs the question why?