The labels left and right are avoided because left-handed people will have the buttons reversed. What does it mean when a lefty mouse has its right button clicked? Should the program perform its right-click action or its left-click action. If we simply swap the mappings, then right and left become rather meaningless to the programmer.
I assume the designers of Squeak wanted to avoid this thorny issue, so actions are labeled with colors which are agnostic to right/left.