A paragraph from this web site.http://entertainment.howstuffworks.com/how-to-play-roulette.htm

Because the next player to use the same color chips may designate a different value, roulette chips have no value away from the roulette wheel. The cashier's cage will not accept them. When ready to leave the table, place all remaining roulette chips on the layout and ask the dealer to cash out. The dealer will exchange them for the equivalent amount of regular casino chips.