Why is my black background making the video layer colours dull? Opacity is 100%

Hi I’m trying to do a simple animation. The goal is to use a black background and then have white glowing lines on top, however the problem is I can’t get the white to be actually white. I’m using the RGB colour scheme and it’s not in grey scale mode (as I’ve tried multiple colours, they’re all dull). I can’t figure out which opacity is causing the issue - all are set to 100% (brushes/layers). The colours are fully opaque when the background layer is deleted but as soon as it’s added the colours start blending with the black. I’ve set the blending mode to both Normal and Linear Dodge (makes it slightly lighter grey) and I’m now all out of ideas. I need the black background for this animation. Can anyone else offer any suggestions? Or any idea why the black background is blending with my video layer? I’ve attached a screenshot.