Paint cross section from the upper left background
This paint sample was taken from the area above and to the left of the sitter, where the x-ray shows that Gainsborough initially painted a tree before changing the setting to an interior.
In the magnified cross section you can see the pink ground layer at the bottom. Over that reds, browns and yellows with an upper red glaze provide the russet tones of tree bark. The green layers of the curtain have been applied over the tree trunk.