It really depends on the aplication
modern Sterns use a crappy blue insert, putting a blue LED under those, makes them reallly stand out

I also used cool-white LEDs in my Iron man under the Mark 1-V inserts as they look closer to what tony's arc reactor looks like in the movie
I also used differnt colored LEDs under the ramp inserts cause they were boring

on Spiderman, I used green LEDs as GI lamps under the Goblin
I also used Red LEDs under the slingshots ansd flipper feederlanes, combined with those etched web plastic protectors, it gives the area a different look
I used yellow LEDs in the GI near the Sandman, but that was a waste of effort