What flavor of gray do you think you want? A true, neutral gray is achromatic, is colorless, has no trace of hue. And is actually kinda hard to find as far as paint colors go. What most people mean when they say they're looking for a gray is a chromatic gray. Chromatic grays are so close to a true, neutral gray that it's difficult to detect any hue, but they do have some degree of hue and do indeed belong to a hue family. You can find a chromatic gray from every hue family in the visible spectrum. Depends on what you're looking for. Examples of hue parents and lighter/darker, grayed child colors:

I would paint the stairway in Marilyn's dress. It's better to have a lighter color for basement stairs, and it will flow with both rooms but be darker than your current white. You can paint it in an eggshell finish so fingerprints wipe off more easily. You also may be able to paint just the handrail side of the stairs in Blue Danube and the other stairway walls in Marilyn's dress. Or for more of a statement, consider a patterned wallpaper with colors that incorporate both the gray & blue. Do you have photos of the spaces?

You might also look at BM Winds Breath. Can be really nice but it's more of a greige than beige.

Looks Beautiful ! What paint color did you choose. I need to choose a color for family room and kitchen ASAP. I hope the Mega finish holds up well compared to Traffic… wonder if I should have upgraded.