When it comes to an open or arch way I suggest figuring out which side is going to be viewed the most, but as suggest it's a matter of what looks good to you.
Anytime your going to any dark color from a light one, to prime the walls first with primer that's been tinted a dark gray, and never buy cheap paint. If you skip one of these tips, plan on having to paint at least 3 to 7 coats to get the wall looking so the color evens out.
Time after time I've suggest this to customers and they skipped the priming with gray step to save money or time then try to paint a dark blue or burgendy color over white then end up spending more money on paint and two or three days painting this one wall.
Clean, then sand the whole wall with a sanding pad on a sanding pole, that will find all the flaws in the wall and old paint runs before you paint so they can be fixed before painting.
If you have not done much painting before, do not use blue tape, it's expencive, you will end up getting paint under the tape or peeling the new paint off the wall. Buy quality roller covers and a Purdy 1-1/2 sash brush and learn how to use it, it will save you a lot of time. Cheap roller fall apart and leave lint, a cheap brush will leave paint lines and the brushes fall out.
You only need to use the first third of the brisles. Do not wipe the brush on the side of the can, just tap the brush on the side and twist it as you move toward the wall to stop it from dripping, Do not have a full brush when trying to cut in, wipe some on the wall about an 1" away then work you way to the trim.
Paint the ceiling first, paint all the trim next, then do all your cut in around the trim and inside corners before trying to roll the walls.
More info then you asked for, but so be it.