We tried a concrete stain on our slab floor in the new house we're finishing now, without good results. I did acid etch first, but the color just wouldn't take. I'd strongly recommend trying in an unobtrusive spot before you commit (closet, maybe?) I have a splotchy partial stain under the bathtub now, but fortunately no one will ever know.
I ended up just sealing the floor in the whole house (slab on grade), and I'm actually quite happy with it. If you're looking for gray anyway, you might try a section with a stain and a section with just sealant in that closet.
Pick a warm spell of weather, even for the test; you're going to have to open up the whole house, trust me. I had fume issues even though our south wall wasn't fully sheathed yet, and that was just with the sealant.
I used Eagle brand sealant if you want to look into it. It gives protection and a nice shine without being slippery, plus a slight darkening of the gray. I don't recall the stain brand, but I can check if you like.