Everyone is absolutely right. Other colors must be dry before applying tape. BUT - cool secret - after applying the tape paint along that edge with the color that is under the tape you are trying to protect. It will fill in the gaps and save the color under! Let it dry for about 30 mins then go ahead and roll on the color you want in that section. As soon as you finish the room remove the tape. Nice clean crisp edges. :-)
P.S. Last color you paint in the room (taped off section) should be the darkest color as well.
