>>23664
>glow effect
*screen blending
You don't want it to "glow". You want it to screen blend in with whats behind it (as rainbows do).
The best way to test this is find a good photo of a rainbow, and try to replicate it in Photoshop. Blend modes are helpful for this experiment, because you can easily replicate them in graphics shaders.
The problem with glow effects, is that they often use additive blending, which will end up looking like a smudge of white mess (kinda like bloom, but way worse).
Additive blending does certainly achieve the whole (make stuff behind the rainbow appear brighter) look, but it also crushes the blended colors into white far too much. You don't want that, as you want your rainbow to be colorful yet retain its initial detail.
To achieve a good rainbow, get a vector of some colored lines and apply a Gaussian blur effect at around 2.5 pixels.
Then render it to a billboard with a shader performing screen blending, at about 60% opacity.
More information:
https://en.wikipedia.org/wiki/Blend_modes#Screen
http://elringus.me/blend-modes-in-unity/