These colors were first suggested by Peter Coad, Eric Lefebvre, and Jeff De Luca in a series of articles in The Coad Letter,[1][2] and later published in their book Java Modeling In Color With UML.
That is, attributes, methods, associations, and interfaces are fairly similar among classes of a given archetype.
Many people feel colored objects appeal to the pattern recognition section of the brain.
That extra dimension allowed modelers to see important aspects of the models (the pink classes, for instance), and to spot areas that may need reviewing (unusual combinations of color classes linked together).
It was also easy to review the standard types of attributes, methods, and so on, for applicability to the current domain effort.