system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In the photos you see of nebulas, galaxies etc. What exactly does the coloured smoky areas represent? Is it just light or something cooler?
gpt
Those are clouds of gas.  As a quick and dirty guide you can break then into three groups.

Red ones are thicker regions of hydrogen that are warmed by nearby starts to glow a red color.

Blue have dust in them and are reflecting light.

Dark ones can actually have larger dust grains, molecules of hydrocarbons etc.  They are blocking most light from passing through.