Was this color work on primitive film at the time? It looks it. Actual tintable color film has been around forever, filming onto it was the breakthrough in the late '30s.

The coloring does look original. At that time this type of Sunday comic was done for one paper only, not syndicated. McKay and the plant supervisor stayed up all Saturday night monitoring the colors during the print runs. This is why they are so beautiful: the readers got it that gorgeous in print.