I hate answering like this, but I think your premise is flawed - there does not seem to be a radical shift from mostly bold colors during the war years to mostly pastels after them. Light colors were popular in the 1950s because they had been popular in the years leading up to that decade.

There may be a perception that there was a shift based, perhaps, on the difference between black-and-white 1940s films and vividly-colored 1950s movies, but black-and-white film greatly skews the viewer's interpretation of the color of the original clothing(/upholstery/wallpaper/etc.). But the 1940s and 1930s were also colorful decades that made use of pastels, as pattern/fashion magazines of the era will show, and 1950s clothing was often boldly patterned.