When a strong colour is needed
I have tried to add white lights to the rear of my star wars imperial hauler engines, and pop on some blue Lego plates to change the colour to blue, but wasn't very effective, and still came through as very bright albeit a pale blue, adding these blue led's really gave the effect that I was looking for which is pretty much on par with the movie footage that I was using for comparison.
still using a blue lego plate on top of these blue led's gives a great effect (see additional photo)
this moc is still a work in progress and I've yet to add additional lighting to the display, just waiting for parts from lego.com.