Card counting is not illegal in Colorado or any other state, as it is not considered a criminal activity. However, casinos have the right to ban players who they suspect of counting cards. It is a skillful strategy used by some players to gain an advantage in certain card games. The legality of card counting is a gray area, as long as no external devices or assistance are used. In Colorado, like in other states, it is up to the individual casinos to decide their policies on card counting.