There is a fundamental problem with glows. **Glow cannot be merged into a composite with alpha**, because such layer is then composited using the **'mix/normal'** color operation. What you need is having the glow **separate** and combine it with **add/dodge**. Here is the explanation:

[![enter image description here][1]][1]

If the *Glow layer* (foreground) is lighter than *Background*, the Normal/Mix color operation with alpha will not be as bright as original Add operation with alpha. If the Background is lighter than the glow (a color glow on white) - the end result will be **darker** than *Background* leaving unwanted effect!

That's why you **never ever ever** composite color glows into a single merged layer with alpha that's supposed to be overlayed with Mix/Normal mode over some other than complete black background.


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Now let's say you have object with a color glow on **black** background and you need to make this background transparent. Here is how you correctly do it:

[![enter image description here][2]][2]

You break down the image into channels and assign each channel as it's own alpha - what this is is **separating** the background **per channel**. As it is black the alpha of the background is 0.

Next you color each channel of it's own color and you add them together to reconstruct the image. Simple.

With nodes:

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/eUD6I.png
  [2]: https://i.sstatic.net/YvSqT.png
  [3]: https://i.sstatic.net/XDlUV.png