I would use two tones of the same color. Yes bottom of chair rail darker than top, but not a huge contrast. Not sure what you mean leaving baseboard wood. Can't see in pic. I would paint all the dark brown trim/ a clean white. Then decide the colors you want. Your pretty wooden dining table would look nice w yellow walls. I use SW Rising Sun on top and SW Lemon Chiffon on bottom and I have antiques. It doesn't look dated and lets the antiques pop.

I would paint it all one color, and remove the chair rail.