See a good urologist. 
When I was a kid, I once bought some “disappearing ink” at some toy store or somewhere. It seemed to be water colored very dark blue, like fountain pen ink. However, after I spilled it on something, the color disappeared completely in a few seconds, leaving a damp spot. I think someone told me the color disappearance was because of an oxidation or bleaching reaction.
Maybe a magic store would have something that is similar to this in the color you want.
You could always use elemental bromine. It has the following characteristics you are looking for:
- Deep orange
- Liquid
- Volatile
- After it all evaporates, it leaves no residue.
However, it has the following disadvantages
- Acrid smelling
- Highly corrosive
- Reactive with most metals
- Toxic
You’re choice…