Citadel Mechanicus Standard Gray Base paint is designed to give a smooth, solid dark gray color over an undercoat and for that reason this will almost always be the first paint you use on a miniature. Shake the dark gray paint before using (you can make this easier by adding an agitator ball).
The dark gray paint can be a bit thick at times and then you just add a few drops of water to your pallet. You really don't need a lot of water. Something to watch out for is to make sure you don't apply too much paint so that you don't start hiding details.
Another important functionality of this dark gray paint is that it not only covers the undercoat, but also covers other colors. So if you later miss something about this color, you can easily paint over it with this Mechanicus Standard Gray base paint.
The color of Citadel Mechanicus Standard Gray (21-24) is similar to: Old Citadel Adeptus Battlegrey; Vallejo Model Color Russian Unif. (70924); Instar Steel Grey; Reaper Master Military Green 2; Privateer Press P3 Great Coat Grey.