Are you sure you are not modifying gs.Color.White at some point?
It is quite easy to do that unexpectedly:
var = gs.Color.White
var.a = 0.25 # will actually modify the constant (sic)
Here is one way to avoid that:
var = gs.Color(gs.Color.White) # explicit copy from constant
var.a = 0.25 # modify the copy, leave the constant intact
We'll make sure to get constants read-only for the next release.