I personally find yellows in artificially-lit spaces tricky.

In terms of transitions, without some pictures its really hard to say.

I sometimes like a small or dark place like a foyer or hallway to be painted to retain that moodiness, as long as it is light enough to see. That way walking into the lighter, larger space is a "destination" and a bit of a lift.

I think there area a lot of things you could do but it depends on the "feeling" you want to give the space. Modern? Traditional? Wild?

Also, I would want to try and find a color upstairs that I could pull into the stairwell or landing so that it transitions well. You said the walls upstairs are Vanilla ice cream (yummy) - but what are the accent colors in the upstairs rooms. Any bright warm blues? Reds? Oranges? Light greens? Warm colors are great for small dark areas. Esp trimmed with pure white. I would avoid pastel or greyish tones. So little light will only make those colors more dull. IMO

I'm personally partial to yellow. My northern facing small den and kitchen area are much warmer and feel brighter painted yellow; more than when they were beigey-white. And yes, it took a LOT of samples to find one that worked in every light and corner of the room. But it works (all the trim is bright white.)

The yellow follows one wall up into the landing space of the stairs, but then the wall of my VERY dark stairwell is bright white with a sheen to make it reflective. I did hang artwork on the landing to pull the colors together. One painting on the yellow wall and one on the white wall - both the same artist, feel, and related colors. I like it.

Another thought, is to incorporate stripes (I'm getting into stripes lately. You could do the foyer in a warm color, but on the wall that extends upstairs (and maybe one other) paint bright white horizontal stripes. That gives you color but lightens it up. And the horizontal shape will help make the space look bigger. On another thread, as an example, someone posted this link of aqua and white stripes. http://www.thehandmadehome.net/2010/06/the-veranda/

Another idea is to keep it all the same color as upstairs, but add marvelous art and lighting fixtures, and maybe jazz up the risers of the stairs. Here's another post I found in FW that has some GREAT ideas you might be able to use:
http://ths.gardenweb.com/forums/load/decor/msg071758089676.html

Here is a link that might be useful: aqua stripes

ohhhh... here's some neat ideas.

Here is a link that might be useful: bunch of foyer designs

I think your foyer/entry should reflect the colors "to come" in the rest of the house. What colors do you have in your living area? If you have blues, then a blue would be appropriate (I seem to say this all the time on this forum, but BM Healing Aloe and Quiet Moments are great colors). If you have greens, then maybe a green... you get the idea. In other words, don't just pick a random color that has no relationship to the nearby rooms.

Either way, I'd go fairly pale and airy if it's a small/cramped space.

For the transition upstairs, not sure how to advise without seeing the space.

These are all great suggestions. Thanks Karen for your input! I like the idea of stripes but it wouldn't work with the rest of the space upstairs, I'm afraid. But I love the idea of a bright rich color with bright white molding, though there is molding only at the bottom (baseboard, actually). The ceiling in the stairwell is verrry high so putting molding IN would not really work either *sigh*

The colors upstairs are a pale, bluish grey carpet, yellow Corian kitchen counter, the Vanilla Ice Cream walls and pale maple kitchen cabinets. Therefore, I'm thinking Sueb's idea of trying Healing Aloe or QM would be great... but I want to test some pretty yellows too since upstairs it could tie in as well. Btw, have any of you bought the "full spectrum" version of these (or other) colors? As in this website below that I stumbled upon today in my searches? (it has some beautiful examples!)

Here is a link that might be useful: full spectrum paint

Oooh, Ellen Kennon's Buttercream (suggested on the other thread you linked me to!) could work perfectly! It's not too yellow yellow, and Ellen herself has it paired with a bluish light gray on her site, and my carpets are greyish. I wish it was available retail so I could run out and get a sample now...

Here is a link that might be useful: Buttercream