It really depends on how sharp the corner is. If it is somewhat rounded off, it's hard to just roll it and make a straight line. You may need to use a brush or tape and create a straight line yourself. If you do use tape, you should use the blue and press firmly down the edge where the paint can bleed under onto the other color. You can then use the color that is under the tape first against the tape to seal that edge there so that there won't be any bleed when you use the real color.

Thanks, paintguy. The corner is fairly sharp. I was wondering, with the tape, if it would work (and look OK) to put the tape just a hair away from the corner? Like I've seen suggested for the corner of a ceiling, where the paint line is actually either on the wall or the ceiling. Or would that be too noticeable on a wall?

I've used the trick before, of using the color under the tape first, to seal it. That does work well, I hadn't remembered it for this case.

As long as the line is straight, I think it would work fine.

I have several outside corners where the paint color changes. My technique is to hold the brush such that I paint toward the corner. Just don't overload the brush with paint. So long as you don't permit the brush to wrap onto the adjacent wall, you will get a clean edge without using tape.

Now inside corners - that's another topic!

Good luck.

I found a solution which works amazingly well on pinterest. Tape a line with blue painters tape as normal. Then paint OVER the tape the same color as the wall it's taped to. This will seal any gaps under the tape with the wall color. Then once that dries paint again using the color on the other wall. Once dry peel the tape back gently and you have a straight edge. We've repainted almost every corner in our house it looks so good. Good Luck!