Josef Albers was one of the greatest artists and teachers of the 20th Century. He taught a color course at Yale University's School of Art. His students learned that color appears to change depending on the colors that surround it.
This is an Albers painting: Albers combined four reds: a bright one, a deeper one, an even deeper one, and the deepest one. Notice that the reds seem to curl up. Albers did not mix his colors. He used colors directly from paint tubes.
Albers said that if different people chose what they think of as "red" each person would choose a slightly different red. There are many different reds. Here are some studies from a student in the Albers color course.
By placing blues that are a little lighter in the center, they seem transparent and appear to float above the deeper blues.
Which color is the background color?
Here is a project for you. Look for some reds in magazine photos. Cut the reds out and create a collage by putting lots of reds together. Paste them down with tiny dabs of glue. Look at all the reds. and choose which one you think is the "real red?" Ask other people to pick their "red." Same one? Different?