One question I'm often asked is where to stop paint colour. Many people are unsure about how to paint odd corners or shared walls within a space. Even the nicest colour can look wrong when it's applied in an awkward way. Here are a few guidelines for a couple of different scenarios:
If the ceiling in your room goes up on an angle from a low wall, continue the wall colour upwards. Painting the slope the same colour as the ceiling (usually white) brings the ceiling too far down and makes the room look oddly proportioned. Carrying the wall colour up visually raises the wall and avoids making the space look like it's been chopped in half.
This can also be applied to bulkheads -- especially if the ceiling in the room is lower. Painting the bulkhead the same colour as the wall gives the illusion that the wall continues up. A bulkhead coloured to match the ceiling brings the ceiling down lower and shortens the wall instead.
When it comes to rounded corners, stopping the colour would creating an odd line, so colour should be continued. Only stop at straight corners/edges.
A common scenario is the living/dining room combo. When both rooms share one long wall, that wall should be one colour -- stopping anywhere along that wall will look choppy. If you choose, you can paint the entire wall a different colour from the other walls in the room to define space -- but that wall needs to be all one colour along its whole length.
Applying these guidelines when painting can help keep your room from looking out of proportion, and will make that great paint colour look its best.