Variables in fvwm are turned into their value at the time the code is read, so after the code is read by fvwm, the variable is no longer a variable, but a value given at the time the config was read. If you want to change the colorset at a later time, you have to send fvwm the full colorset command with the changes, just changing the variable will have no affect of the colorset in the future.
It wasn’t to change it during run time. It was more to standardize the colors. So if I made a change to default bg color, it would change all of the colorset occurances. More like a #define statement in C.