Sometimes you get it right the first time, and in this case, I think the neutral beige color for the walls is a good decision. If it were my house and I wanted a change, I would be looking at adding end tables, lamps and changing the wall decor.

The green carpeting is the limiting factor here. When the carpeting goes, then they could rethink the wall color.

While I agree that the green carpeting is very limiting, I think more contrast is needed between the sofa and the wall to warm up the room and pull it all together. Right now the sofa kind of fades into the wall color .. fine if you're going for a monochrome look, but it doesn't work with the carpet. She might consider a darker shade of the sofa color to make the room cozier.

A brownish tan. Favorite Tan by Sherwin Williams is a nice one.

I think something with a reddish color in it like rust tones would work the best as it's the complimentary color to green. Here is a color viewer where you can upload your photo and show your your parents their own room with a variety of colors. Let them decide and then you won't have to listen to them the rest of your life lol Joking, of course.

Here is a link that might be useful: color viewer