Citadel Mephiston Red Base paint is designed to give a smooth, solid red color over an undercoat and for that reason this will almost always be the first paint you use on a miniature. Shake the red paint before using (you can make this easier by adding an agitator ball).
The red 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 to hide details.
Another important functionality of this red 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 Mephiston Red base paint.
The color of Citadel Mephiston Red (21-03) is similar to: The Army Painter Vampire Red (WP1460); Old Citadel Mechrite Red; Vallejo Game Color Heavy Red (72141); Vallejo Model Color Red (70926); Instar Crimson Red; Reaper Master Clotted Red (9134).