I'm going through some trials and issues installing LED's on my two machines. I'll probably write up some lessons and advice and put them in the LED sticky thread once I get through everything. I can tell that the final results will be worth the effort, but I'd be happy to spare other people some of the learning experiences I'm going through once it's over.
Meanwhile, in keeping with this thread, I agree that putting color matching LED's behind colored inserts is the way to go. The colors look deeper that way - really eye-catching without being overwhelming.
As far as the white inserts go, putting colored LED's behind them works great. I have one machine (POTC) with too way many white inserts in the center of the playfield. Not attractive at all. I put colored LED's behind them, and it really makes a big difference. The colors aren't as deep as the ones with colored inserts, so there is a little difference in the shading of the colors, but they look fantastic. You may want to keep some inserts white (with white LED's), depending on how they look with the rest of your playfield, but it all depends on what works best for you. White LED's are available in both cool white and warm white. It's easy enough to experiment to see what you like best.