Wikimedia Commons is a great place to start, everything is public domain or freely-licensed. Searching for "color" brings up some good ones, like these:


They run monthly photo contests (which I've been meaning to post here) that can get weirdly specific, and which might be useful to you. For example, One Stands Out might be a good test of image processing techniques:
https://commons.wikimedia.org/wiki/Commons:Photo_challenge#June_2025

The Wiki Science Competition gets some really interesting photos, the winners are collected here:
https://www.wikisciencecompetition.org/
You can see the current list here and past years here:
https://commons.wikimedia.org/wiki/Commons:Wiki_Science_Competition_2025