It's this method: https://www.damiensymonds.net/preventing-banding-in-backdrops/
Here's the file: https://max.mailbigfile.com/c5103403247a5566ab8dbf71675651d5/listFiles.php
Ok, good.
I URGE you not to try to change its colour. Just add a smooth layer of the colour that's already there, to save yourself the hassle of tedious masking.
By the way, are you aware your photos are Adobe RGB? This is VERY bad.
I ... don't understand what you don't understand.
The two little squares on top have to be solid black all the time. They are the opacity of the colours.
The two (or more) little squares underneath are the actual colours that we see. They are in the same position as the histogram - shadows at the left end, highlights at the right end.
Yeah, lots of people ask about this. Ultimately you'll have to decide for yourself how you approach it - the whole speed vs cost thing.
I urge you to take the Bridge Class. It's SO important for this.
Anyway, we're getting off track. I'm glad you've ordered WHCC prints. Once they arrive, you can calibrate your monitor again to ensure a perfect match.
Then PLEASE dive into your Raw Class!