Are you using the Desktop (installed) version of Hemingway, or the web app? That will be the first difference that needs to be known.
Then, having looked at their webpage, are you asking for NVDA to report the background color applied by Hemingway automatically when you're moving through your document, or just to be able to order NVDA to tell you the background color on demand? [And given the nature of Hemingway, I'm not certain that either of these may be possible depending on what it exposes to NVDA.]
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Growth for the sake of growth is the ideology of the cancer cell.
~ Edward Abbey