The locating agent is ninhydrin.
Ninhydrin is 2,2-dihydroxyindane-1,3-dione.
Its structure is
Ninhydrin is a powerful oxidizing agent.
It causes the oxidative deamination of alpha amino acids to aldehydes, ammonia, and carbon dioxide.
The ammonia then reacts further to form a blue-purple coloured substance.
However, the cyclic amino acid proline reacts differently to form a bright-yellow product.
Hence, when you spray a TLC plate or a paper chromatogram, the colourless amino acids suddenly appear as bluish-purple (or yellow) spots.
The colours variy slightly from acid to acid, probably because the unreacted acids complex with the pigment.