I saw tag list:image composition now, but I don't think any of the backgrounds listed there fit the post you mentioned.
Posted under General
Danielx21 said: We don't have abstract background.
We could start using it I guess.
Color tags aren't really done in any systematic way because there are so many colors, it's mostly just someone thinking "hey I see a lot with the color, let me tag it". We don't have a list of "core colors" we allow in tag variations or anything.