I have a runebook of locations where I've found frenzieds spawning. It makes tracking down specific colours or stats much easier as you can cull the ones you don't want, then move onto the next spot instead of waiting around for a respawn.
As far as I've noted, the colours seem to cycle (forest ostards spawn in 2 colours at once, I think red+green and yellow+blue from memory) and get darker as they go. The gold ones eventually turn up brown, the greyish ones eventually become the "shadow" hue, a very dark blue.. and so on.