Agreed with MysteryMeat. That background has too much contrast, just about any character would be easily lost in it. I recommend having some character sprites in the scene on a separate layer as you work on it, so you can easily gauge how well they stand out.
You could tone down the contrast without losing those lovely blues or any details though, you'd just need to use a narrower range of values, something like this:
I don't have the exact palette you're using so the colours aren't exact, but hopefully it conveys the idea.
There's no escaping having a darker background overall if you want the neutral areas to be solid black while also having the characters overlap the backgrounds. If you have both the solid black and a bunch of light colours in the background, that creates too much contrast. A solution might be to replace the background black with a lighter colour in those areas where you want a "light" feeling:
(I think this is still too much contrast though, unless the characters are all super-dark.)
Whatever the values of the background are, you generally want the overall value range to be narrow, so that the characters aren't lost in the high-contrast areas. You could also use darker colours as outlines in some cases, but this can also create high-contrast nose, so you should be careful with outlines. In general, if you want more value range in your backgrounds, you have to put the extremes in areas where characters won't touch them.
Digging this tileset, by the way!
