The 1960s was a turnaround in style. It's when I say fashion and art really entered the modern age. Bright colours stopped being garish and started to become fashionable. Geometric shapes, block colour and new never-before-seen fabrics walked out onto the catwalks. Twiggy became a fashion icon, famous for the traditional 60s minimalist styles.
So, all these new styles started, and inspired artists such as Andy Warhol and Roy Lichtenstein. Transforming traditional fine art into bold, bright, fun pieces of art. Lots of them only used one or two colours, one for the background, and one for the main detail. Then black and white for further detail. Repeating patterns were also a hit, by replacing the main colours each time, and putting the pictures into a grid.
I think that this was some sort of rebellion. Against the feminine shapes of the 50s, against the technology free days of the past. They wanted to show off all the new found modern stuff that had brought them forwards.