I think your idea of charcoal grey is a really good solution. Do you have a photo? It may look good but you may be having a buyer's "remorse" or worrying for nothing! I think we have all at one time said "What have I done!" lol

Forgot to add the pictures.

Well, it looks fine to me but computer screens are lousy for a color critique. I suggest you get some charcoat paint, paint a poster board the has been cut to the size of your splash and tape it up and take a look.

Or perhaps you need to add an additional backsplash above the 4" one you have. The new splash could be a glass mosaic will a combination of the colors in the countertop and in the cabinets. That will help you tie them together and add some interest and sparkle to that area. Below are some pictures of mosaics that might work. But DO NOT buy any mosaic tiles from just a photo. You'll need to go look at tile and bring samples home to check them out properly.
Carolyn Albert-Kincl, ASID