It's hard to tell from a photo, especially considering it's on the carpet, but when it's blood it's like little streaks of red in normal urine. When it's uniformely red, it's marking pee. Look to see if one of your rabbit seems to be in pain while urinating.
Blood in urine is really rare. It is generally caused by uterine tumors (a possibility if you've got an unspayed female over 3 year old, but considering you've got 2 bunnies I'm going to get you don't) or a UTI.
Colored urine, from orange to violent red, is pretty common. It can be caused by some vegetables (carrot...), some plants (dandelion...), some hay (Crau hay...) or by a desire to mark (mine have always done that on soft surfaces, like a bed, a cushion or linen... I guess they find them comfortable and really want to keep them!).
If you've got any doubt, go to a vet.
