Marc Jacobs was born in 1960s, which was inside the complex of inter-related cultural and political trends which occurred roughly during the years 1956–1974 in US. Therefore, hippies and counter-culture emerged.
1960s was also known as the decade of he new, exciting, radical, and subversive events and trends, including fashion culture. Drugs also became popular among youths, especially for show biz. Pop, rock, reggae, soul and blues music developed rapidly. This period of time gave Marc Jacobs a nice foundation to absorb novelty fashion concepts and add these elements in those classic products.
For 1970s to 1980s, they was probably the most influential period to Marc Jacobs, as he was a teenager and absorbed a lot from the surroundings.
1970s was still in the complex years and hippies culture continued. Besides that, the whole Western countries experienced an economic recession and the living standard of people dropped. That may give us a clue that why Marc Jacorbs has great interest in the 40s' and 50s' fashion trends, comparing to the items in his decade.
In 40s and 50s, World War II affected the whole world in economy, culture and social aspects. Dark colours and stand sleeves always appeared in western fashion to create a military and safe feeling. Moreover, to create sense of uniformity, regular forms, balance in both structures and colours are emphasized. For balance in colour, Split complementary color scheme and Tetradic (double complementary) color scheme were prefered.
These key elements appears in Marc Jacobs 2009 S/S collection with Marc Jacobs' own modifications, mainly in cutting and styles of usages. For colours, he seems to like using Split complementary colour scheme with different in shade and value.
Here are some examples below.
1/2 proportion & contrasting colours
Photos: http://07176470d.blogspot.com/ (groupmate)