Because the Imaginauts are traveling inside and interacting with paintings, we need to make sets for where they travel (obv). The ImagiNATION is a place where paintings and artists can interact with each other and the scenery blends multiple artistic styles together. The paintings and artists are ghettoized, so paintings by one artist are generally found near or around each other, and paintings and artists with similar styles tend to be geographically located near to one another as well.
There are certain hot spots that are popular for characters from paintings to hang out. Famous paintings are celebrities and hot spots within the ImagiNATION. Think of the Mona Lisa as someone who would be like Madonna in pop culture.
To make each world a little distinct, we should use specific colors to differentiate them. I bet Rococo would be pink and pastel green! Also, how the world is depicted depends on the style of the paintings that exist in it.