I just started noticing this error yesterday and was trying to debug it when someone else reported it in Zulip.
Notice: Undefined index: base in _color_rewrite_stylesheet() (line 691 of /app/src/core/modules/color/color.module).
It seems like this error messages started appearing after the most recent core update and @joseph_flatt speculated the following:
I think that the issue is happening more because of the change so that we regenerate color files when clearing caches. The error is coming from the fact that the color module requires that one of the color sets is named 'base', and then it generates it every time the files are regenerated.
You should try commenting out the 'blend_target' setting from your color.inc. We made color blending optional in a release a while back, and the color blending is what is looking for fields with a specific name.