Making deep, velvety darks is a challenge for many watercolor painters. Part of the problem comes from the unavoidable fact that the paint lightens as it dries. Of course, we all know this will happen, since it always does. If a wash looks dark enough when you first lay it down you can be sure that it will not after it dries. We just have to get into the habit of compensating for that reality by making our washes a little darker than we ultimately want them to be.
But there are other obstacles not so easily overcome.
We have all observed that adding another layer of paint makes colors darker. What about making layer after layer until the shape is dark enough? What's wrong with that technique? Unfortunately, multi-layered darks usually look streaky. It's difficult to cover a fairly dark area with additional layers without disturbing the previous ones. Paint that was already attached to the paper gets lifted and repositioned through the movement of the brush, causing uneven saturation. The more layers it takes, the likelier there will be patches and streaks in the finished wash. It may be dark enough, but only at the expense of depth and evenness.
The reason layering gradually darkens a shape is because with each layer you are adding more pigment to the area. Why not get all the pigment you need in the first layer? For one reason or another, we do not want to acknowledge how much paint it really takes to make darks like Andrew Wyeth, Piet Lap, or Mary Whyte. Every time I have worked with someone struggling to make their darks dark enough they are surprised to see how much paint they have to squeeze out of the tube. Every time.
Stump Andrew Wyeth
It is not easy to make a big enough puddle of very saturated paint, either technically or emotionally. I often see painters struggling to make a small patch of color stretch to cover a big shape, which is a sure recipe for too many brushstrokes. The paint has to be fluid enough for each stroke to merge with the others in the wash without leaving overlap marks.
At the same time, it has to be thick enough to feel truly deep. How can we satisfy these mutually limiting requirements, wet enough to flow, but thick enough to cover? Try starting with the right amount of water. You can measure with the brush you will be using to apply the dark wash. Ask yourself, "How many brushes full will it take to do the job?" Add a little extra so you won't run out. Then begin adding pigment to the puddle of water:
Even more! (Here's where the emotions come into play - fear, shame, denial!).
The range of what counts as transparent watercolor is wider than most us think. We are shy about making the paint thick enough, almost as if it were impossible, or sinful. Self-imposed limitations like this are tough to even see, let alone transcend. If you are worried about making the paint too dark, do it! Go into that forbidden territory. It's the only way to discover where the real boundaries lie.
Draw a simple shape on good paper.
Estimate the quantity of water you need to make a wash that will cover the shape.
Add pigment (not water!) until the value is dark enough and the puddle is as thick as heavy cream.
Apply the paint to cover the shape with an even wash.