I found someone doing this and I wondered how you do it in blender 2.8. Preferably in cycles as I need to texture bake the final result.
I'm willing to add more details if needed.
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It has nothing to do with the painting per se.
Make sure to use black as blank color when creating the texture, so it doesn't emit light when nothing is painted.
I don't use the Principled BSDF's emission here because there's no strength input.
The glow itself comes from his viewport which is in rendered mode:
And using Eevee which has a bloom effect that can be seen in the viewport:
You can do the same with Cycles, except it doesn't have any SFX visible in the viewport, you would need to do it through compositing using a glare node in fog glow mode: