So my painterly side is humming, and I love talking about gray and the magic of the color gray. So forgive me for throwing in some five cents that may help (or may not) but I think the issue is just that gray is hard and deceiving, especially when trying to get it to match with pops of orphaned color spots in your panels. But, gray can totally work here.
Gray has color to it, but it can only appear to have color if it is next to a contrasting gray. I do feel like you already put color into your grays and did think about making a warm gray vs a cool gray--but it's so subtle that it's nearly the same gray.
So like, when I look at your image, I think the issues aren't resolved with a screen layer because you're just moving all the colors in one direction, including the grays that were very similar on the base layer. I think it's the base grays of the bg and their clothes being so similar in midtones that you aren't getting a contrast so they blend together and orphan the brighter colors in their hair and clothes. I'd just stretch further what you already are doing with the contrasting hues in your gray and seeing if it works? Maybe that'll help?
Again feel free to disregard what I'm saying, I might be totally off of what the problem is that you are seeing. Your work does look pretty nice so this is mostly nitpicking.