How Do I Ensure That the Background I Remove By Smart Lasso Is the Color I Want?

I saw your video of how to remove a background in Preview. I do use the Smart Lasso, but I find that in order to get the right color background, I have to cut and paste the image onto that color and then take a screenshot of the whole thing. I note that in your video, the background that you remove is automatically grey. How do I change that color to the color I want?
I want my cut-out from Smart Lasso on a certain color background, and I want to know how to get that color when I delete the background sections I don’t want.
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Gail