while software will get you so far, you still need to put some paint
up. 3rd eye had a good list of factors to consider.
If your certain on the primary wall paint then go ahead and paint those
walls -- or at least one wall that's next to the accent wall. the
bigger the sample the better. then start testing colors for the accent
wall -- get a test quart [or 5] and put a big sample on the wall. or,
paint a big piece of poster board so you can move it around. the accent
color can be as dark as you want, though i'd suggest that the darker
the color the more you'll need to light it up.
if you're doing trim only around the accent wall, or in a room, do
whatever you want. if it's the whole trim package -- meaning, all the
trim in the house -- i'd stay close to white.