Books with title Little Blue and Little Yellow: a Story for Pippo and Ann and Other Children

Author's first book written on a train to entertain his son. NY Times Best 1959 Illustrated. Series of dots running through book showing children how combined primary colors become another color.

Little blue and little yellow share wonderful adventures. One day, they can't find one another. When they finally meet, they are overjoyed. They hug until they become green. But where did little blue and little yellow go? Are they lost?

A little blue spot and a little yellow spot are best friends, and when they hug each other they become green