Citadel Castellan Green Base paint is designed to give a smooth, solid green color over an undercoat and for that reason this will almost always be the first paint you use on a miniature. Shake the green paint before using (you can make this easier by adding an agitator ball).
The green paint can be a bit thick sometimes and then you just add a few drops of water on 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 green 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 Castellan Green base paint.
The color of Citadel Castellan Green (21-14) is similar to: Old Citadel Catachan Green; Vallejo Game Color Cayman Green (72067); Vallejo Model Color Olive Gray (70888); Instar Jungle Green; Reaper Master Muddy Olive (9034); Scale75 Boreal Green Matt.