most common is both colors the same around a true chair rail, but making the lower one darker would probably mimic wainscotting, which is usually close to trim color...

here's a dining room that goes darker below the chair rail:

and here's the same in a hallway:feels strange to me, but I guess it depends on your look -- light "wainscot-look" is probably more traditional, but it all depends on your exact colors and other choices.

just look at a bunch of photos and see what you like. I suspect that lighter colors overall will help the hallway feel wider...

i like darker below unless below is painted wood, and then it would be done trim color. if both are painted wall, dark on top looks top heavy.

Thank you all for your comments. At one point I read an interesting article that said when you use 2 different colors there is an optical illusion that suggests that the hallway is actually wider than it really is. For that reason I usually use 2 colors when doing hallways to bedrooms or upstairs hallways which tend to be narrower. Thank you again.

My clients are both artists and I know they will want to hang their artwork on these hall walls so I am thinking that a lighter top wall would be a better background for their art. Darker below to give good contrast and give the illusion that it is a wider space.