Bright pink blood
This is what we want to see! You've probably hit the heart or lungs. If there are bubbles in the blood, odds are high it is a lung hit. Follow up immediately but cautiously.
Dark red blood
Often indicates a liver hit. Wait several hours before trailing.
Not good, especially if there is some grainy material on the arrow shaft and/or fletches. Often indicates a hit through the digestive tract. Back off quietly and wait at least half-a-day. If you shot the deer in the afternoon or evening, wait overnight before continuing the trail — and then be on red alert for the deer to still be alive when you arrive.
Dark red, with soupy consistency
Often indicates a muscle hit with no internal organ damage. Old school thinking was to wait overnight before following up, but the new school — to which I subscribe — is to push a muscle-hit deer as hard and fast as you can, hoping it will either bleed out or you can get another arrow into it.
When making the shot, be sure to concentrate on the little spot on the animal that you wish to hit and follow through. At the same time, try and see exactly where the arrow hit the deer. When things are happening fast and you arrow is flying nearly a football field a second this can be difficult, but knowing where the arrow hit is a big help in the tracking process. That's one reason light-colored fletches, lighted nocks, and video cameras have all become very popular.