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Answer 1 · 2017-07-24T16:01:04

Proline gives a yellow colour because it is a secondary amine.

Most amino acids are primary amines with the general structure $"H"_2"NCHRCOOH"$ .

Except for proline and hydroxyproline, all the α-amino acids are oxidized by ninhydrin to give the same intensely colored purple anion.

The central $"N"$ atom is the only portion of the purple anion that comes from the amino acid.

Ninhydrin
(Adapted from Biochemistry is a good thing - WordPress.com

Proline reacts differently because it is a secondary amine.

The product is a yellow-coloured zwitterion.