The dog playing Laddie (the son of Lassie) is actually Pal - the dog who played the original Lassie in Lassie Come Home.
This film was the first filmed in the Technicolor "monopack" process, where one magazine of film registered all three primary colors, rather than the original three-strip Technicolor process (introduced in 1932), where a separate magazine of film had to be exposed (and processed) for each of the three primary colors.