Citadel Khorne Red Base paint is designed to give a smooth, solid dark red color over an undercoat and for that reason this will almost always be the first paint you use on a miniature. Shake the dark red paint before using (you can make this easier by adding an agitator ball).
The dark red paint can sometimes be a bit thick 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 to hide details.
Another important functionality of this dark 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 Khorne Red base paint.
The color of Citadel Khorne Red (21-04) is similar to: The Army Painter Chaotic Red (WP1142) (too dark/brown); Old Citadel Scab Red; Vallejo Game Color Scarlett Red (72012); Vallejo Model Color Carmine Red (70908); Instar Wine Red; Rackham Arcavia Red (35); Reaper Master Deep Red (9002); Scale75 Deep Red Matt.