We have a newer home and all of the exposed drywall corners are rounded making it difficult to have rooms of different colors. This is especially a problem in our dining room which has arches on both ends (with rounded edges), leading into the living room and kitchen. The interior was completely painted in one color of contractor orca white.

Any suggestions on how to paint rooms of different colors with rounded edges? I want it to look good without adding molding or building corners back in.