Interesting, I bought a set of both cool and warm grey copic markers to try and get a mixed grey slightly duo-tone look instead of just straight up one-tone grey. It works well, but I was very surprised at how much the warm grey will make the cool grey look "blue" on color. I didn't tweak the colors any. It's a nice effect if it's what you're going for.
But it is another good reason NOT to mix warm and cool greys in the same area on a drawing!