what color will be produced by proline with ninhydrin? why?

Answer 1

Yellow
All other common amino acids have a primary amino group. The primary amino group is essential for the formation of the purple chromophore.
Proline has a secondary amino group so the reaction is different

Answer 2

Just to add to Jesse's reply, which is correct, is that all the amino acids excepting proline give purple color upon reaction with ninhydrin. Proline is an exception because its amino group is involved in the ring formation and hence it is technically an imino acid, and hence, the difference in color