the shallow meaning: they are the primary colors in squares.
the "deep" meaning: we are all living things, and consequently, we are all energy. our eyes filter thousands and thousands of multiple vibrations through three colors: red, blue, and yellow. therefore, we aren't seeing reality as it really is. we are merely hearing a song with three different keys. this is a reminder to myself that my visual perspective isn't as it always seems, that there are so many colors that my eyes are missing and so many different vibrations I cannot detect.
(I prefer to use the first meaning, because it's short and sweet and not as complicated)
by the way, that mark in the yellow square is a birth mark. I'm probably going to turn it into a smiley face or something random in the future.