They're talking about a design perspective. People like the lime green toaster because it by itself looks nice, but if they placed it in _their_ kitchen, it would probably look absolutely ridiculous unless they're designers and know how to perfectly balance such a strong color in the room.
In contrast, neutral colors will look good in any color palette and require zero design talent from your side.
We're talking about the context of this "study". If people are asked which toaster they want to take home, the vast majority of them will have a kitchen where "grey, steel, white or black" works the best, even if they prefer another color. They're not going to redesign their entire kitchen for a particular toaster.
I don't understand that argument. My toaster only needs to work in my kitchen, not every kitchen.
From a manufacture's point of view I get it. It is cheaper to produce 3 colors than 54.