What Is the Golden Rule of Watercolor?

Work light to dark, and leave the white paper alone. That is the golden rule of watercolor, and unlike most things called rules in art, it is not a preference, a tradition, or a school of thought. It is a physical property of the medium. In transparent watercolor you cannot paint light back on top of dark. The white in a finished painting is not white paint — it is paper you never touched. So the lights must be decided before the first wash, and protected through every layer afterwards.

Everything else people call a rule is a matter of taste. This one is a matter of physics.

Why it is physics and not opinion

Opaque paints work by covering. Put a light stroke over a dark one and the light one wins, because the pigment is dense enough to block what is beneath. That is what opacity is. Oil, acrylic and gouache all permit you to change your mind about where the light goes, at any point, for as long as you have paint.

Transparent watercolor works by the opposite mechanism: light passes through the pigment, strikes the white paper, and comes back up through the pigment to your eye. The paper is the light source. The paint is a filter.

So every stroke you make is a filter placed over your only light source — and filters do not un-filter. You cannot make a passage brighter by adding anything, because everything you can add makes it dimmer. The brightest a passage will ever be is the moment before you touch it.

Read that again, because it is the entire discipline: the brightest a passage will ever be is the moment before you touch it.

What follows from it

Almost everything, as it turns out.

Plan the whites first. Before mixing a single colour, know where the light lives — the highlight on the cheekbone, the rim of the vase, the sparkle on the water. Those are not things you will add. They are things you will avoid, deliberately, for the whole painting. Many artists mark them faintly in pencil so they cannot be forgotten in the heat of a wash.

Build in layers of increasing darkness. Lightest washes first, over the whole passage. Then the mid-tones, into the areas that need them. Then the darks, last, and only where they earn their place. Working this way means every stage still permits the next one. Working the other way means you have closed the door before you reached it.

Treat every dark as irreversible. Because it is. This is what makes beginners so frightened of them, and why so much amateur work lives timidly in the middle of the value scale — nothing dark enough to make the light glow, nothing light enough to make the dark necessary.

The part that hurts

The golden rule means that a watercolor is planned before it is painted, and that offends something in most people. We want to discover the painting as we go. We want the freedom to change our minds. And this medium says: you may change your mind about almost anything, except the light — and the light is the painting.

So when I look at a student's watercolor, I can very often tell you whether it was thought or hoped. Clean, shaped whites sitting exactly where the form turns: that painting was thought before it existed. Highlights that are scrubbed back, chalky, or rescued afterwards with opaque white: that painting was hoped through, and the paper is telling on it.

This is also, incidentally, how you judge a watercolor in a gallery. Look at the whites. They are a map of the artist's intentions from before the first mark.

Is there any way around it?

There are tools. Masking fluid preserves whites mechanically, and it is legitimate — though it solves a problem better solved by planning, and it has a habit of bonding permanently into paper whose sizing has failed. Opaque white and gouache exist, and every honest painter has reached for them. But they announce themselves. A gouache highlight sits on the painting; a reserved one glows from inside it, and the eye knows the difference even when the mind cannot name it.

You can break the golden rule. You simply cannot break it invisibly.

The rule beneath the rule

Work light to dark. Protect the whites. But if I could give a beginner only one sentence, it would be the thing underneath both:

Decide where the light is before you begin, and then spend the entire painting defending it.

Everything else — the washes, the edges, the timing, the restraint — is in service of that. Watercolor is not a medium about applying colour. It is a medium about protecting light from your own enthusiasm.