From dark to light: the reverse order

If you've ever worked with watercolour, one rule sits in your fingers: the paper is your lightest value, so you leave the highlights untouched and work layer by layer into the dark. With opaque paint, exactly the opposite applies. Acrylics and gouache cover, so you can lay light over dark. That reverses the whole process – and this reversal is precisely where most beginner sketches in opaque paint fail, not the material.

The underpainting: away from the white sheet

Before you do anything else, tone the whole sheet in a mid, muted value. Diluted burnt sienna, a neutral grey or a mixture of both work well. You're not trying to set a colour, you're trying to get rid of the glaring white, because on a white ground every value you lay down afterwards looks too dark, and you correct yourself into a dead end.

The underpainting may be uneven and patchy; it should show through at edges and give the picture cohesion later on. Brush it across the sheet with a wide brush in twenty or thirty seconds and wait until it's matt. With acrylics that happens very quickly outdoors, with gouache a little more slowly. In both cases the rule is: don't keep working into a layer that's still glossy.

The dark masses first

Now look for the darkest areas of your subject – not the details, but the masses. The shadow side of a building corner, the depth under a canopy, the area beneath parked cars, the interior of an open window. Lay these areas in as connected shapes, with relatively dry, barely diluted paint.

Three or four dark shapes are usually enough for the picture to hold together. They're your framework: everything that comes afterwards is measured against them. If you set the darks too late, you won't dare go properly dark at the end and you'll end up in a grey middle mush with no contrast.

Midtones as the largest share

The bulk of any subject lies in the midtone: façades in half-light, road surface, tree canopies, sky in overcast weather. You lay these values opaquely over the underpainting, shape next to shape, deliberately leaving small edges of the underpainting showing. That saves time and keeps the sketch alive.

When mixing, always compare each new value with the darks you've already laid down, not with the palette. On the palette almost everything looks too dark, on the sheet almost everything looks too light.

Highlights last, thick and sparing with white

Only right at the end do the highlights come: the sunlit edge along a roofline, an illuminated sign, the bright side of a façade, a reflection on wet asphalt. You paint these spots thickly and opaquely, with little water, in short, decisive strokes. If you hesitate and go over them several times, you'll tear open the layer underneath again.

Mix white sparingly. The more pure white there is in a value, the chalkier and deader it looks. A highlight is almost never white but coloured: in sunlight you mix white with a hint of yellow or ochre, in shadow under a blue sky or at dusk, white with a little blue. You make this decision – warm or cool light – once per sketch and carry it through. Warm highlights next to cool shadows (or the other way round) create a sense of space almost automatically.

You make edges with the brush

In watercolour you control soft transitions with water. With opaque paint that would be a mistake: water makes the paint transparent and muddy. Here edges come from brush handling. You set a hard edge with the edge of the brush in one stroke, without touching it up. You get a soft edge by scrubbing across the not-quite-dry boundary with an almost dry brush, or by interlocking the two values at the seam with a few dabs. Less water, more wrist.

Two mistakes that keep happening

The first: painting too thinly. If you dilute acrylics or gouache heavily, you no longer have an opaque painting but a murky watercolour without the luminosity of white paper. The paint should come off the brush like cream, not like tea.

The second: too many layers into paint that's still wet. Every further layer laid into a glossy area mixes with the one beneath and produces mud. Let each layer go matt before you continue, and limit yourself to three or four layers in total.

Exercise

Find a simple building corner outdoors or from a window where the light and shadow sides are clearly distinguishable. Paint it twice, small, about eight minutes each. The first time deliberately wrong: start light and work your way into the dark as in watercolour. The second time correctly: toned underpainting, dark masses, midtones, and finally the highlights thick with white plus a hint of yellow or blue. Put them side by side and look at which version has more contrast, cleaner edges and clearer light. Note in two sentences what came more easily to you on the second run.