Choosing grey paint is a cinch. You only have a few...thousand choices. Do you want a light grey, medium grey, charcoal grey? A brown-ish grey, blue-ish grey, black-ish grey? It's enough to throw your hands up and just keep the color you've got. Or, check out some real homes with grey walls and see what colors and brands they used as a starting point for your own paint project.
You know this, but it bears a reminder: variables in photography and your computer monitor make it difficult to precisely replicate these colors, so take that into consideration and consider these a good starting point.