Magazines were a printed product and the colors would need to be printed in the preferred color format to show them correctly. CMYK(Cyan, magenta, yellow and black). It was easier to preserve the painted colors rather than the first photographs in print work.
Use of contrast for effects
Using contrast colors of the titles would help the audience to easily identify the magazine used on the motive vs the backgrounds and would make them stand out for people to see who or what is featured.