Ideal spots to farm minions

I'm kind of in a tight spot with Overlord at the moment.

I have to do a water-based dungeon, but my Blues count is awfully low.

Where can I farm each type of minion to get that count right back up when I need it? I know for browns you just massacre every sheep in sight, but where (in what areas, most particularly) do I go if I want Reds, Greens or Blues?

Just figured out, you can just go to the dungeon and spawn the beetles of the type you want minions of. You'll lose a lot of minions, but I managed to get about 40 when I lost 25 minions to the mobs. A good sacrifice if you ask me.