Amphetamine mono-Phosphate must be prepared by titration in water, the ph you want is 5 to 6. Monophospate cannot be prepared in alcohol as when one adds the phosphoric acid 75 or 85% doesn't matter, as is or in alcohol to the base in alcohol there comes the point when the all the Amph turns all of a sudden into bubblegum and it does not take on any acid anymore. So you have a mixture of polyphosphates . It is no problem to dry this bubblegum mass and it gets dry without problems and it can be consumed and its not bad but not the mono-phosphate.
The monophosphate which can be made by adding water and then diluted phosphoric acid to the base in a non-polar solvent and then mix the holy shit shit out of both, has the advantage that it is much more water-soluble then the polyphosphate and also more then the sulfate. This higher solubility means a better bio-availability and it has a slightly faster onset then the other salts. Only the carbonate salt is said to beat it, but I never tried to make it.
Now when you make the monophosphate in water something happens what will disturb you first: When adding the phosphoric acid you will first see salt falling out of the solution and this will make you happy. When adding more acid all of a sudden all the salt will -puff! - disappear again! In difference to sulfuric acid where this is bad, it is exactly what you want with phosphoric acid salts as you want the one with the best water solubility. Which you get at ph 5 to 6 and after separating and evaporating the water.
Red discoloration which appears when adding too much acid is not caused by acid or amphetamine but by impurities, mainly P2P and mildly basic impurities which came through the process. P2P goes nicely through steam-distillations and A/B procedures as it is amphoteric in the presence of bases and acids, in special with bases, say it becomes suddenly water soluble to a far wider extent.
Really clean Amphetamine base does not change color no matter how much acid you add. None of the post-reaction protocols I have seen here produces really clean Amphetamine base and so the reading about the discoloration (from pink to red) problem is no surprise at all.