Opacity is how completely a paint hides what is underneath it. It is one of the more useful properties to understand, because it explains why some colors cover in one coat, why others need three, and why a pale color painted over a dark one so often disappoints.
What Makes a Paint Opaque
Two things, and neither of them is what people usually guess.
The pigment itself. Opacity is a property of the pigment particle — specifically how much its refractive index differs from the binder around it. A large difference scatters light and hides what is beneath; a small difference lets light pass. Titanium white is opaque because titanium dioxide scatters light extremely well. Quinacridone rose is transparent because its particles barely bend light at all.
How much pigment there is. More pigment in the same amount of binder means better coverage. This is most of the difference between student and artist grade paint, and it is why a cheap white sometimes needs three coats where a good one needs one.
Two things that do not affect opacity, despite being commonly claimed:
- Sheen. Gloss and matte describe how the dry surface reflects light. A matte paint is not less opaque than a glossy one.
- How dark the color is. Ultramarine is a dark blue and fairly transparent. Cadmium yellow is bright and highly opaque. Darkness and hiding power are unrelated.
Which Mediums Are Opaque
The medium sets the range; the pigment decides where in that range a given tube sits.
- Gouache is deliberately opaque — it is watercolor with added filler for exactly that purpose.
- Acrylic spans the whole range. Some tubes are labeled opaque, others transparent, and good manufacturers print it on the tube.
- Oil likewise, and the old masters exploited both: opaque paint for the lit areas, transparent glazes for the shadows.
- Watercolor is transparent by design. You work light to dark and let the paper do the lightening.
One correction to a common line: acrylic is not inherently less opaque than other paints. A tube of cadmium red acrylic hides just as well as the oil version.
Opaque and Transparent Pigments
Reliably opaque: titanium white, the cadmiums (yellow, orange, red), cerulean blue, cobalt blue, chromium oxide green, yellow ochre and the earth oxides, Naples yellow.
Reliably transparent: quinacridones, phthalo blue and green, alizarin crimson, dioxazine purple, ultramarine, and most of the modern organic pigments.
In between: zinc white is semi-opaque and much weaker than titanium, which is exactly why it is useful for tinting without bleaching a mixture.
A note on the toxic ones, since older articles list them without qualification. Lead white was the finest opaque white ever made and is banned or restricted almost everywhere for good reason. Cadmium pigments are still sold to artists but restricted in some markets; they are safe on a brush and hazardous as a powder or if ingested. Wash your hands, do not sand dried cadmium paint, and do not point a brush with your lips.
How to Make a Paint More Opaque
- Add white. The simplest method, and it changes the color — the result is lighter and less saturated. Sometimes that is fine, sometimes it is not.
- Undercoat in white. Far better where you can. Lay white, let it dry completely, then put the transparent color on top. The color stays true and covers in one pass instead of three.
- Use less water. Thinning acrylic spreads the same pigment further, which is a common accidental cause of poor coverage.
- Buy a better tube. Artist-grade carries more pigment. For whites and yellows in particular, it is worth the difference.
What does not work: more coats of a thinned transparent color. Ten transparent layers are still transparent.
Why This Matters for Paint by Numbers
A kit's paints have one job the shop-bought equivalents do not: cover a printed number and a printed outline, ideally in a single coat. That makes opacity the single most important property of kit paint, and it is why a kit filled with cheap, thin, over-extended paint is so frustrating to use.
Two practical consequences:
Pale colors will need two coats. This is normal even with good paint — a light yellow simply cannot hide a dark printed number in one pass. Let the first coat dry properly rather than piling on more while it is wet.
A white undercoat saves the pale sections. Thin white first, dried, then the pale color. The number disappears in two coats instead of four, and you use less of the color most likely to run out.
Ours are formulated to cover the printed number in one coat where the color allows it, and vacuum-sealed so they arrive at the right consistency rather than half-dried. If you do run short, there is a matched Extra Paint set for every design.
Buying Paint
Look for the opacity symbol on the tube — most manufacturers print a filled square for opaque, half-filled for semi, open for transparent. It tells you more than the price does.
Liquitex Basics is the usual recommendation for a first set: inexpensive, widely available, and honest about which colors are transparent. Golden Heavy Body carries more pigment and holds a brush mark. Any set will do for learning; buy the white in the largest size you can, because you will get through it faster than everything else combined.
If you would rather not think about any of this, a numbered kit hands you colors already matched to a design — the catalog is here, and Custom Paint by Numbers makes one from a photograph of your own.
Related: acrylic vs watercolor, and passive and active colors.
