Stupid question, but why do you want to buy new Pmags?
You do realize that all Pmags are date stamped with their month and year of manufacture?
If I lived in Colorado and was in the market for new magazines, I would buy magazines that did not have a manufacture date on them. Then it would be (next to) impossible for anyone to prove when they were obtained. But if your magazine has a manufacture date on it, that is post whatever the effective date of the law is, you just proved your own guilt.
I fully realize Colorado is not currently enforcing this law, but it would not surprise me if in 10 or 20 years newly elected sheriffs (who may hate guns having grown up in New York or California) don't send officers to the ranges to check on magazines. The least I would do, if I were you, is buy a few non-dated metal magazines to use at the range on the off chance Colorado starts enforcing the law.
YMMV and good luck with living in an ever bluer state.