I have LEDs in all four pins, almost fully now (GI, backbox, playfield and some flashers too)..
I have some, I believe cointaker (though not sure cheap or better ones?), in a PIN I bought, I pulled them all out and put the pinballcenter.eu NOFLIX PLUS in, I much prefer them as they have tiny capacitors in them so the go on and particularly off like a normal light (not like strobe lights that distract me)..
1. Do I need to colour coordinate to match the insert for best effect (purple insert with purple LED) or can a plain white be put under the purple insert and look just as effective?
I colour co-ordinate to match the insert, you do you get far more colour..
2. Flat Top/Concave/Frosted lenses - does it matter? and any preference of one over the other?
Under playfield I use flattop or flattop on a little movable stem for the lights don't face the insert (90 degree set).
3. LED's for the backbox - plain white or colour coordinated to match the translite artwork?
You can have fun putting colours in but I am lazy and just use the cheaper non PLUS (no capacitor needed as they stay on most of the time) in the warm white...
4. LED's for the GI - yes or no?
I use the frosted top warm white (they are warm but not incandescent warm) regular NOFLIX (not plus as they stay on)..
