I think you're on the right track with a pale yellow, but I have a feeling that the bright accent colors will look better with a cleaner wall color, like maybe Jersey Cream or Full Moon. Ivoire is toned/grayed down and it may look muddy with lime, turquoise, etc. I also worry about potentially muddy colors looking even more so in north facing rooms. I like to keep colors clear and clean in north light. My DD wanted a deep blue room and we went with a BM Historic color, also somewhat toned down, not a clear blue. Instead of the off white and toned down gold I thought we'd be using, she opted for lime green. I think a cleaner blue would have been better for that accent. Luckily, she doesn't give a hoot about those kinds of subtleties and I only go into her room under duress. :-) If you have a Ben Moore dealer near you, take a look at Ambiance. It is a pale yellow with just a touch of pink in it. I used it in my north-facing laundry room and it is very subtle and pleasant. I think it would work well as a backdrop for your daughter's brights...will let them take the stage.

You do not want a gray with blue undertones in a North room. North light is blue and gray and it makes a room look dull or dirty. What will work is a gray with lavender undertones. Try B.M. Slip

I just painted my north facing room smokey taupe from Benjamin Moore. I tried BM Halo on poster board in the room and looked at it from all different angles and at all different times of day and thought I loved it. When the room was painted it looked mint green. I had to then make a decision about what color to repaint and snow made it so I couldn't go get samples -- so I did as much research as I could on other "grieges" and picked smokey taupe from a paint strip because the articles I found said it had red undertones versus green -- and I love it. Has a nice taupe-gray feel that's very neutral. The upshot is, pay attention to what research you can find about undertones -- I don't recommend picking from a paint strip, obviously, but the research helped a lot (and I guess I got a bit lucky). I know seeing is believing, but a little supplemental research can really help, too. Wish I had paid attention to it before I had to repaint!

Have a peep at 868 Mountain Mist by Benjamin Moore. It can be a little ethereal, depending on the light. Pretty. It’s very similar to Farrow & Ball’s Borrowed Light. They are both from the Blue-Green Hue Family.