I'm going with gray walls and flat white ceiling in my shop as well. I just finished painting my cold storage room. It was somewhat of a battle to get the right tone of gray.
My local hardware store had a sale on paint a couple months ago, buy one get one free.. so I bought 10 gallons. The color I chose was called "gray flannel". At the store it looked great. I started painting and did all of the edging-in and by the time I was nearly done, some of it had almost fully dried and turned out to be really blue (see attached pic). Not what I was hoping for.
I took all of the paint back and had it tinted to get a different shade of gray. I learned a lesson from the first attempt, so I just quickly painted a 4'x8' area and let it dry fully before I went any further. The next day I checked it out and it was purple-ish. That wasn't what I was hoping for either.
Third attempt at tinting the paint seemed to work out. Though it's a little darker than I hoped.
I see you had said you have OSB on your walls, as do I. My advice would be to use a good primer to start with. I used Zinsser Bull's-eye 123 primer. It's very thick and covered in one coat. The paint also covered in one coat.
I would also advise to choose your color at the store, but try a small area in your shop and let it dry fully before going to town and wasting a bunch of time only to find out it's not the right shade. Chances are the lighting in the store and the lighting in your shop will vary.
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