I get the impression that when people give out flags, they feel like they're penalising somebody or putting a mark against them. I think that works against the function of flags to bring posts to our attention, since people might feel bad giving flags, and then decide not to flag a post that needs looking at. There have also been occasions where people got angry for receiving flags.
I think a more neutral colour would help shift the perception a bit, and make it feel like less of an insult. It would help people view flags as a way to attract staff attention, not just to 'report' posts for breaking rules. (For example, you could flag a post that needs its grammar fixed, has a bad title/tags, has broken images, etc.)
Something like a light blue, violet or yellow could work.
EDIT: Maybe also change the hover text as well. I don't think any post that is flagged is necessarily 'spam or inappropriate' — it should just read 'flag this post for attention from moderators'.
