Help with a Backlight 5/Lightroom/Photoshop problem

My wife and I enjoy visits to art galleries on our travels. We enjoy capturing photos of many of the works of art and I enjoy creating web albums on our family web site to save memories for ourselves and to share our travels with family and friends.

I have enjoyed using Lightroom and TTG products from the earliest releases for this purpose and have appreciated how much these products have improved over the years.

One of the more recent improvements in Lightroom and Photoshop that make it simple to precisely select objects like paintings is particularly helpful for me in this task. However, I have not been able to figure out how to change the background (after selecting the painting as an object) in order to simply change the background to a neutral gray equivalent to that I use as the background on my web site. That is RGB (125, 125, 125).

This link shows a recent example to illustrate my problem:
Paris 1874: The Impressionist Moment - The Collins Family on the Web:

Photoshop wants to replace the background with a variety of colors and textures (including shadows), and all I want is a gray background that disappears in the presentation of the photo on my web site.

Can anyone with better (probably much better!) skills in Lightroom (and, if necessary, Photoshop) help me accomplish this seemly simple task?

Thanks so much!
Doug

Your gallery background is using rgb 80, 80, 80 (hex #505050)

Are you doing this:

Select > Subject
Select > Inverse
Edit > Fill: Color. Then enter the color code