Issue with convert a QuickTime transparent video with Gaussian Blur "Glow" to WebM

The solution may be very simple, but I can't figure it out. I have a video using CC Star Dust and Gaussian Blur effects with transparent background (AE-Shutter-Encoder-settings.png), converted to QuickTime format, which I convert to WebM format using Shutter Encoder (AE-Shutter-Encoder-settings.png). I would like to use the resulting video as a transition in OBS, but as you can see in the attached picture (OBS-settings-final-res.png), if the background is too white, the Gaussian Blur effect will be grey instead of the red color I set, but the color will be well rendered against the black background.

I have tried all kinds of settings and solutions, but the Gaussian Blur effect always turns grey on OBS.
Thanks ahead for your help and sorry if I missed something to mention.