Contrast effect. It's likely the current color of the walls.

In a perfect world, a neutral gray would frame the samples.

A white border is less than ideal because viewing a color on a white background will make a color look darker than it is, feel bigger in mass and volume, and at the same time weaken its hue - white breaks hue so it looks duller.

Additionally, it's only when the walls reflect off each other that a paint color's character fully develops in a 3 dimensional space. Especially true with colors of white.

Painting bigger samples can help - like from the corner to the window and at least 5 ft vertically. Tenting on entire wall isn't a crazy idea.

First, I hope you used the actual formula/base from BM or SW and didn't color match.

I used Snowbound in a powder room that has a taupe-y colored floor tile and it still looks quite yellow, but much better than the White Dove that was there before.

Personally, I find 4000K lighting too blue/cool for most residential applications. I went with 3500K and find it to be a good mid point.

It is also misleading to test colors with ram-board covering the floors. You need to consider the actual floor type/color.

Because the current wall color has a lot of yellow.

We don't see colors independently, we see them in combinations and our minds compare the colors that we see to one another.

The center block on all of these larger blocks is the same color, just shown on different backgrounds.

What really matters in the end is what the color will look like next to the flooring, cabinets, countertops, trim. . .

They are looking at the color next to a color that is going to be gone. They are going to cover the walls with a new paint color. What really matters is what the color will look like against the colors that are going to be there when the project is finished. They should be determining the wall color on how it works with their flooring, furnishings, cabinets, countertops . . .