How Do i Remove the White or Black Backgrounds VFX I downloaded for free in AE?

Hi i’m a newbie here trying to learn, I downloaded the free assets like dust particles and blood mist. When I dragged them into the composition. It has a white background and the other one is black. How do I remove that? Luma doesn’t work well. Is there no transparent effects I could get to try with?

If it has a black background you could try using the “screen” blending mode in After Effects or the “Add” one.
I don’t know one with a white background but in After Effects you could try things like “extract” (or “luma key” in the obsolete effects) or one of the keying effects for it. Most either have an alpha or can use the screen/add blending mode in AE so you can put them over other layers.

You could also check actionvfx’s youtube channel as they have some tutorials on creating compositions with some of their clips.