Why can't I get opaque white (or any colour) brush strokes over black background?

Hi friends! I'm trying to do a simple animation with photoshop using a black background - the goal is to have white glowing lines on top, however I can’t get the colour white (or any colour for that matter) to be fully opaque on my video layer. 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/blend setting is causing the issue - all are set to 100% (brushes/layers). The colours are fully opaque when the black background layer is deleted but as soon as it’s added the colours start blending with the black it seems. I’ve set the blending mode to both Normal and Linear Dodge to see if that changes anything (just makes it slightly lighter grey) and I’m all out of ideas/YouTube tutorials. Can anyone else offer any other suggestions?

I’ve attached a screenshot and I'm hoping I'm missing something completely obvious. Apologies for long post and potentially stupid question! Thanks in advance!! (Also photo of lines just to illustrate colour issue/not actually the animation in question)

Screenshot of settings