Glowing effect

I've done this for glow, works okay

Paint the thing whatever color it needs to be. Let's say green.

Mix a little white into the green paint, now loosely dab that in the center making sure to leave some of the previous green around the rim.

Then just keep doing that as many times as you want, I do it 3 or 4 times. You're basically dabbing circles inside circles, so the inner circles get smaller and smaller. In the very center you'll be dabbing a very bright version of the color (white with a little bit of green).

Note I'm saying dab, not brush. If you brush you're gonna get sharp circles, and that just looks weird (unless you want a cel-shaded effect). If you dab you get a smoother transition between colors where they meet at the edges.

This doesn't take long, maybe 2 or 3 minutes. Just mix the little bits of paint on the fly.