Right there in the topic, I can't figure it out. You can only choose millions of colors and trillions of colors when looking at the depth in the output module. And even if you pick 8-bits per channel in the render settings it still renders the files to 32-bit.
It's basically a (pure) white spinning star fading out gradually so there is no need to render it with that many colors.
What am I doing wrong?
8bit PNGs don't have Alpha channels, they use palette-based transparency by tagging a specific color to be interpreted as transparent by the "rendering device/ client" (browser or whatever). AE can't produce these kinds of files since it deosn't work this way. You have to use Photoshop or otehr tools to convert your image sequence/ clip.