you can perform the action as usual.
/example: there are already 2 blue dice on the "silk" action space. red would like to buy some silk as well this round. red places 2 dice (4 and 5) onto the action space.
red pays 4 coins (as her lowest die value was a 4). she then takes up to 3 silk and adds them to her warehouse.
when a player wishes to place dice on an occupied space, stack them one on top of the other. this will make it easier to see who is the last player to have chosen that action. if need be (too many dice), place them next to one 