Understanding paint viscosity, and the pro's cons of having ti thick or thin can only really come from experience and practice,
Generally thinner paint is desired for several reasons; its self leveling and wont show brush marks, and wont fill up the details as easily, Also coverage tends to be better with less paint; sounds odd doesnt it!, if you do one big thick gloopy layer of white, compared to 3 or 4+ lovely thinned out layers of white then the coverage will be much better and likely use less actual paint.
For drybrushing, thinning might not be so clever, as making a paint more wet and then trying to do a dry technique is a bit backwards.
To actually go about thinning, put your paint on the pallete as normal, and also put a drop of clean (preferably distilled) water and add water to the paint with a brush a little at a time untill desired effect is attained. Maybe with additives in... flow aid, drying retarder, acrylic mediums.. theres a lot of scope for changing the behaviour of paints, and I cant suggest any untill you know what you want form the paint. Does it dry too fast? add retarder
Do you have to add so much water to thin it that the colour is now too weak? add some flow aid and use less water than before. Need transparent colours for various layering or blending effects? matte medium is handy, want to airbrush? Airbrushing medium, or a solvent thinner will be needed.
Dont just thin your whole pot out, as you wont be drybrushing anymore
Edit; as a rule of thumb... citadel colours are not suitable for painting models right out of their pots, every colour is miles too thick and needs thinning a fair amount before any quality paint job can become possible really.
I had to actually laugh in the face of the guy running my local
GW when the new paint line came out... he was really excited and insisted i try the new ceramite white base paint out because the coverage was apparantly just tooo good to be true. When i asked him if he has any distilled water, flow aid, drying retarder and matte medium on hand so I could actually paint well anything he looked at me gone out! And suggested that I paint it straight out of the pot, at which point I burst out laughing at him. Demonstrating to him what a difference even using tap water makes... I based one of the stores starter session marines with thick gloopy white and the eyes and riubs of all the cables dissapeared, but it was seemingly solid white, then I painted another instance of the exact same figure with ceramite white thinned out with clean tap water untill it was like milk, applied a very thin and patchy coat, followed by 2 more after each layer was dry, not only was every last little detail still clear, the white was actually noticably more white and cleaner looking than the gloopy model done in the way the fools behind the counter is teaching kids to do it! :/ then i showed him how much of a non improvement this is over skull white which comes out basically the same only is natrually a warmer white tone.
