If everything fell into place for the color until you got it on your walls, maybe, just maybe, you'll luck out and it's something simple like your light bulbs ;) For best 'true' color, try a full spectrum bulb. Verilux and Chromalux are a couple I've used. (Menard's sells them if you have one in your area, as do some health food stores near me.) Expensive ($7 bulb) but they last for years (no kidding, I get 3 years on bulbs we use in the family room/evening time). Next best would be GE Reveal. See if that doesn't take the yellow out of your cream paint.

If bulbs are not the issue, I'd definitely get some samples of the lighter 'fix' color and put on posterboard, to make sure it doesn't yellow in the room. Get a sample brochure of Whites, and steer clear of those with heavy yellow undertones (you'll easily spot them next to the other whites). But you'll still need a warm white, rather than a cool white (pink, cool gray, or minty undertones) so it doesn't fight with the yellow color now. Couple thoughts. Provence Creme is now your basecoat. You want to tone it and lighten it. You can use the Woolie (super easy) and blend two colors over your PrCreme: ProvCreme at 50% (mix some white in with it) in one tray) and then straight white in a second tray. Take a wide brush, make strokes of each color on the wall, blend with Woolie (instructions on box and online).

Below is a photo of a bath I did. Trim color (bottom of photo) is basecoat color, a mid-level taupe. I then used a Woolie to apply lighter taupe (50% basecoat mixed with 50% white) and pure white. (Colors applied with Woolie are mixed with glaze, at a ratio per the can.)

Second thought: you could do nice wide stripes that are so popular now. Just be sure the Prov Creme has set well, apply wide blue painters tape to mark your lines, and paint stripes in white.

Final thought: mix some white with glaze, and use the Woolie to blend it around on the wall for a marbled effect. Or you could do a 'color wash' with white. I've not tried it, but it's a nice watercolor type effect. Very subtle.

Here's the bath done in three shades with Woolie (as a last minute idea I took the tip of a wool duster, dipped just the tip in pure white, and with a quick flip of the wrist flicked it on the wall to create the white 'wispies'). But before that, it was a nice, very subtle blended effect.

Hadn't heard of the Woolie. I'll look into it. Thanks for the suggestions. I'll let you know what the end result is. Any suggestions for open floor plans? I really want something other than builder's white. I'm tired of looking at it!

Have a look over in Home Decorating forum here, click on Gallery. There are lots of pictures there, many open floor palns, especially see the thread for neutrals labeled "Taupes and Browns". (See link below, they list paint colors with photos). Maybe you'll get some inspiration or a starting point there ;)

Do check out the Woolie, it's one of the easiest faux tools to use (don't spend/waste money on the video or extra 'fluff'). Good idea, as with any faux technique, to buy a couple scrap pieces of drywall at HD, etc. and try it out first, till you get a feel for it.

Here is a link that might be useful: Home Dec Gallery Forum