09-09-2018, 05:45 PM
I had a quick look at youtube +gimp +flare and there are too many, some manage to drag out to 20 minutes. 
Back to basics. You might be able to do something like this, depends on your flare.
Have you tried removing the background colour using Colours -> Colour to Alpha ?
As an example using the standard Gimp 2.10.6 lensflare Filters -> Light & Shadow -> Lens Flare on a black background.
and maybe change the colours using Hue Saturation A couple of screenshots: https://i.imgur.com/tvlQoWt.jpg
Now using Colour to Alpha changing the default white to black. There are two sliders, Transparency Threshold and Opacity Threshold. Adjust to suit and ok. Another couple of screenshots. https://i.imgur.com/5GhqLDh.jpg
You do not get this in any of the old tutorials, colour-to-alpha in Gimp 2.8 was all or nothing.
Back to basics. You might be able to do something like this, depends on your flare.
Have you tried removing the background colour using Colours -> Colour to Alpha ?
As an example using the standard Gimp 2.10.6 lensflare Filters -> Light & Shadow -> Lens Flare on a black background.
and maybe change the colours using Hue Saturation A couple of screenshots: https://i.imgur.com/tvlQoWt.jpg
Now using Colour to Alpha changing the default white to black. There are two sliders, Transparency Threshold and Opacity Threshold. Adjust to suit and ok. Another couple of screenshots. https://i.imgur.com/5GhqLDh.jpg
You do not get this in any of the old tutorials, colour-to-alpha in Gimp 2.8 was all or nothing.