Yupyup, it’s pretty simple so hopefully I can explain it ^^’

Got a bunch of images so I’ll put it under a thing :O

First just take whatever it is you want glowing-

Next either lock the transparency and draw in whatever colours you want, or create a layer on top and clip it to what you want glowing and fill that in-

(I forgot to highlight the bit in PS, just check the icon of the chequered square)

then just proceed as normal :) if you have something like this happen where the colours look a bit weird when your layers are styled-

then you will have to either change the colours on one of the layers, or not use layer styles and just select all colours manually like this-

which can be done the same way as before by either locking transparency or making a clipping mask.

I hope this helps! :3

Hi :3 in SAI the layer style is called Addition, and you should be able to find the blend modes box just above where you make a new layer in your layers box:

hope that helps!

_{oh and if your version of SAI is like mine and has a couple of unnamed or corruptly named blend modes and you can’t find Addition or Add then it could be one of those :O}