Seems like I recall something about taxes and flight crews for Colorado. In the old days you were officially based at DFN (flight ops new hire student pilot ) or some code like that until you were released to the line. Once you completed training you became based where ever your bid was located. This only affected new hires as transition pilots were not based at TK during training. Perhaps this is the state tax you pay while officially based on CO and you can file to get some of that returned if you live out of state.

My memory is fuzzy on this as I thought it was something that popped up in the late 90's. I recall there being ALPA updates regarding this back then but since it didn't affect me I didn't follow it too closely.