We owned a split-level for many years. Personally, I think it is best to stay with the same neutral color throughout the adjoining areas. The shade of neutral is very important, as the overall effect of different whites and different creams varies. I ended up with a pinkish white because pinks seem to improve my mood.
You do, however, need to add color somewhere. One way is to paint an "accent wall" in a dramatic shade (or at least a darker shade). Again, I prefer staying with neutral versions of colors. There is a huge difference between, say, an electric blue and a neutral blue. Too much gray makes the color dull, so I try to avoid those. For wall paint, I think it's best to think of it as a background color. Then add paintings, pillows, glass bowls, etc. that stand out well from that background color.
Do you have any leads on a decorator who could help you with color choices? It can be well worth the money. When we built a new house, our builder had a decorator who came into our old house and re-decorated for us. Wow! The paint colors she chose for both houses were absolutely wonderful, and nothing we could have come up with for ourselves. The trick is finding someone who has a good eye, as I think part of it is inborn talent.