Nitro already supports Finder tags when using the file system. However, Apple made changes in Tahoe that I had to deal with.
If you tag an image in Nitro, it writes the tag to the Finder. If you tag in the Finder, the tag appears in Nitro. The tags and colors that Nitro uses are the ones you have set up in the Finder.
The app could write keywords as tags, but that would be a different feature which would not be using the tags you have in the Finder. My point is that the simple case is easy to do, but there are edge cases that get a little strange. For example, if you have a Finder tag called “Portrait” and it’s green. You keyword an image with Portrait. Do you want the green tag in that case, but the other keywords wouldn’t get colors assigned? I’m guessing yes, but I don’t really know.