No one has mentioned Hydrogen peroxide yet???
That' s the best and cheapest tracking tool I' ve found yet. When in doubt over a red/brown spot on a leaf, give it a squirt of peroxide. If it foams up- it' s blood (or some sort of animal fluid) if it doesn' t, it' s just a natural discoloration.
It will even foam days later.

Another thing I' d like to add-(not that I' m any sort of expert)- if it' s a marginal hit and you know it- don' t quit hunting after the shot. Chock another arrow and " hunt" (be still in your stand) for a while after the marginal hit. It was mentioned earlier in this thread and it' s happened to me. Once on the blood trail the buck made a large arc and wound up withing 35 yards of my stand, within plain view. I at the time had my feet propped up and was reading a book. If I had been " hunting" on the stand waiting while the clock out ,I might have spotted him spotting me and saved myself a 350 yard tracking job-(yes I got him!!)
Best I can figure he came back to see what made him hurt so bad.