Here's a site where the decorator redoes a dining room using Yarmouth blue, it's the third photo. I think a creamy warm white would be best. Look for one with a hint of yellow as it will compliment the blue so well.

Here is a link that might be useful: yarmouth blue dining room

Thank you for the link, but I'm terribly hopeless; I need color names :) Whatever works with Wedgewood Gray, Woodlawn Blue would be good, too.

How do I know which whites have slightly yellow undertones?
BM White Dove
BM Navajo White
BM Linen White
SW Alabaster

And do I use a different white for the trim and molding or just go semigloss?

Navajo White and Linen White both have yellow undertones. I'm not familiar with the others you list.

I use BM very vanilla with blue which reads white but is very warm and yummies up a soft blue. Here is a link of all whites and you can understand the warm and cool ones. If you look at the warm the decorator has used it with blue. I'd use vanilla ice cream on the ceiling and the woodwork but you could use white dove if you prefer a very white tone.

Here is a link that might be useful: best whites

What do you mean by 25% Yarmouth Blue? Is that just for the ceiling or are you planning that for the walls also.
Since you have wainscott on the bottom of walls, I think you could go full strenght YB for both the upper walls and ceiling as long as you have crown molding done in a creamy white to seperate walls and ceiling. It will look lighter on ceiling.
I would then do all the molding and wainscott inb the same velvet finish creamy white.

For sure, full Yarmouth Blue on the upper walls. I was thinking a 25% on the ceiling and creamy white on the crown molding, wainscot and trim. I'm worried that full strength Yarmouth Blue on the ceiling will make the whole room look "top heavy." Rather, I thought I wanted the ceiling lighter (see below)

Here is a link that might be useful: raise the ceiling

nosoccermom
OK, I read that link you posted and maybe you would be better off going with the 25% on the ceiling. What is your window situation in the room.

Two double hung windows right next to each other, so a total of 72 inches of window, facing west. Ceiling is only 8 feet high. The room has three door openings, 2 regular size (kitchen, hallway) and one very large to the living room.