Does anyone know the name of the chemical which is a colourless dye, until it reacts with the Amino acids on your skin? Which then causes it to turn a purple colour?

Well phenolphthalein is used to test for the presence of blood if thats what your talking about. The process is not quite as simple as is shown on shows like CSI however. First the sample is obtained then a drop of alcohol is added then several drops of phenolphthalein then a few drops of hydrogen peroxide if blood is present the sample turns pink.

 

It works because a reaction with the hemoglobin in the blood causes the peroxide to oxidize the phenolphthalein not because of any reaction with amino acids.

Edited January 20, 200916 yr by bob000555

Diliute solutions of silver nitrate (which are colorless) leave impressive purple stains wherever they touch skin, but I can't really recommend smearing them on yourself.

 

Henna (lawsone) and the juice from black walnut fruits (juglone) are nice skin-reactive orangey-brown dyes that aren't toxic if you want temporary coloring.

Edited January 21, 200916 yr by UC

the chem you`re after is likely to be ninhydrin.