You also should consider the present molding in the room, the entry door molding [the bottom molding just has to 'blend with the white', but if you have molding up at your ceiling and it is white, then you don't really want a darker gray chair rail — to many colors and a chair rail is visually related to the other molding because, as far as window and doorway moldings it abuts them.

Is the 'white' a gloss white? If the white is of a gloss level, then the chair rail should be white [use the same gloss white or if there is still a higher level, then go for the higher level gloss for the chair rail]. Repeat question. Is the soft gray a gloss? if it is a higher gloss than the white, then the chair rail should be soft gray [the same gloss or higher level as the soft gray]. Darker shade of gray??? This is interesting. If both the white and soft gray are of the flatter versions of paint, then a very high gloss darker shade of gray might work very well. If the soft gray has some gloss in it; consider a only slightly darker shade of gray for the chair rail. [not to annoy you, but if the white is gloss and you feel there is too much white in the room [curtains, etc] then for the chair rail pick a gray halfway between the soft gray of the wall and the white. [mix a spoon full of each together and you'll see what I mean.] Don't be limited. What is the floor/carpet color? [not the main color of the carpet, but one of the subsidiary colors] might make a nice accent color for the chair rail. Are you having any framed paintings on the walls? what are the frames like? You might want to replicate the frame colors in the chair rail, thus internally framing the room.

Definitely don't paint the rail and molding white. A deep shade of gray, or a deep shade of wine would look very good. Also, a slightly deep shade of green. Even possibly black, depending upon your tastes. Good luck.