I just noticed that, looking at persons note edit history, a whole rainbow of colours can appear on the left side.
And I can't find anything on what those colours could mean.

Any info please?

Nobody knows.

Good question.

The short answer is that they don't mean anything, it's just something we inherited from the danbooru code, but never bothered to change.

The longer answer is that the notes page takes the ID of the post and calculates a (not exactly unique) color based on that number. I believe the intention of this is to break up the lines and make it slightly easier to visually distinguish one post's notes from another. I don't look at the notes history enough to say whether it helps for this or not, but I suspect you aren't the only one confused by it.

Hah, I actually thought it was the ratings for the post.