Color theory: the Color Assistant wheel and gamut mask

This article explains why Color Assistant arranges colors as it does, and what Value Check and the Gamut Mask actually help you judge. It concerns digital colors on a screen and the organization of a painting, not a formula for mixing pigments. For button-by-button instructions, see the Color Assistant guide. For a short definition of a single term, use the glossary.

1. Separate three dimensions of color

You can change a color in at least three distinct ways:

Dimension	What changes	Control in Color Assistant
Hue	Red toward orange, blue toward violet: the color family	Move around the wheel
Saturation	Near-gray toward vivid	Move outward from the center
Lightness	Dark toward light	Move the vertical slider

These answer different questions. Making blue darker is not the same as making it grayer. Changing red to green need not move it to another place in the painting's light-and-dark structure. What painters call value structure is how lights, midtones, and darks organize the subject and space; it is not a single number in a color code.

The gray at the wheel's center is not always the same gray, either. At low lightness it approaches dark gray; at high lightness it approaches pale gray. Hold the slider still before comparing hues and saturation within one layer of the wheel.

2. Digital values and the color you see

Digital images commonly store red, green, and blue channels. A HEX color code writes those values in hexadecimal: #FF0000 has maximum red and zero green or blue; #808080 has equal channels and is neutral gray. The three numbers alone do not tell you how light a color will appear. You also need the color space that gives them meaning.

Color Assistant's wheel targets sRGB, the common RGB space used for digital images. A different display range, monitor profile, or surrounding background can change how the same values appear. The wheel gives you a consistent set of picking coordinates; it cannot guarantee identical appearance on every display or against every background.

There are also two ways to sample a reference. The full-screen Color Picker reads the displayed screen pixel. Picking directly from an image on the Reference Board reads the source file pixel, converting an embedded ICC profile to sRGB when present. Board opacity, zoom, and grayscale display do not change that original-image sample. If the two paths return different values, decide whether you wanted the screen appearance or the image data.

3. Why OKHSL instead of ordinary HSV?

In HSV, V is the largest of the three RGB channels. Pure yellow #FFFF00 and pure blue #0000FF both have V = 100%, yet yellow usually looks much lighter. Ordinary HSL's L is not a reliable measure of perceived lightness either: equal L values do not guarantee equal-looking grays.

OKHSL starts from Oklab perceptual lightness, then arranges selectable sRGB colors in a convenient circular wheel. The vertical slider fixes that model's lightness; turning hue within one lightness slice usually gives a steadier sense of value than HSV or HSL. Radius represents relative saturation: the outer edge is a strong color still inside sRGB for that hue and lightness. The same radius at different hues therefore need not mean exactly the same physical chroma.

This is a tradeoff, not a guarantee that every color on a ring looks equally bright under every condition. Background, display, viewing environment, and grayscale algorithm still matter. Try changing hue at fixed lightness, then check the result with your eyes and Value Check.

4. What the gamut mask includes

Here, gamut mask borrows the painting idea of a limited palette range: place a polygon over a wheel to plan where the colors in one painting will be concentrated. This is separate from the gamut a monitor can reproduce. You cannot infer the mixing behavior of physical pigments from this digital polygon.

Color Assistant treats the mask as a soft guide. Colors outside the shape are dimmed, but you can still pick and send them to your painting app. Rotating the shape changes the main hue direction without changing its proportions.

Shape	Arrangement on the wheel	One possible use
Triad	Three directions about 120° apart	One dominant hue and two smaller accents
Complementary	Opposing directions about 180° apart, with restrained sides	Strong hue contrast while keeping other colors less vivid
Split complementary	A main hue and two hues beside its opposite	Contrast without relying on just the directly opposite hue
Analogous	A narrow region around nearby hues, about ±30°	Variation within a close hue family
Atmospheric	A low-saturation region near one hue	Restrain the palette so a few accents stand out

These are candidate ranges, not proof that a painting is harmonious. Evenly distributing colors by angle can still produce a picture with no clear hierarchy. Area, lightness, saturation, and neighboring colors all change the result.

Generate Palette samples a dominant midtone from inside the mask, adds a darker and a lighter version, then takes accent colors from the shape's vertices. Near-duplicates are removed, leaving up to six swatches. Use them to start a painting; remove, replace, or subdue any swatch rather than spreading all six equally across the canvas.

5. Why Value Check offers three kinds of gray

Value Check temporarily displays the screen in grayscale to take hue out of the judgment. It does not edit the painting file. But there is no single way to turn color into gray. The current client offers:

Standard: a weighted screen-RGB calculation matching the Windows grayscale filter. Green contributes most; blue contributes least.
Desaturate: the midpoint of the largest and smallest RGB channel values, similar to Photoshop's Desaturate or reducing HSL saturation to zero.
Perceptual: an Oklab lightness estimate, displayed as a neutral gray of comparable perceived lightness.

These are the current client's 0–255 outputs for a few pure colors. A higher number means a lighter displayed gray:

Input color	Standard	Desaturate	Perceptual
Red `#FF0000`	76	128	136
Green `#00FF00`	150	128	211
Blue `#0000FF`	29	128	85
Gray `#808080`	128	128	128

Desaturate maps all three pure colors to 128. That is useful if you want to match a painting app's desaturation check, but it hides their different apparent lightness. Standard and Perceptual separate them by different amounts. No single grayscale method replaces all visual judgment. If you compare against an app's Desaturate command, choose Desaturate; to examine perceived separation, try Perceptual. Keep the same method while reviewing one painting so an algorithm change does not look like a change in the artwork.

6. Apply it to one painting

Imagine a warm lamp lighting a subject against a cool shadow:

Establish the values first. Before deciding which yellow or blue to use, see whether the subject separates from the background in gray.
Choose a dominant color range. Rotate a mask toward the intended warm hue. Decide whether the cool color is a broad support or a small accent; the mask cannot decide area for you.
Change hue at fixed lightness. Use OKHSL to find warm-light and cool-shadow colors, then deliberately separate their values instead of relying only on a yellow-blue contrast.
Generate and edit swatches. Keep the colors that serve the hierarchy. Sample a reference to correct them if needed.
Recheck with one grayscale method. If the subject still reads when the image is small, the value structure is doing useful work. Return to color and assess the mood and hue contrast.

The wheel and mask help you control color choices. Lighting, material, area, and adjacency still determine the final image. These tools help you experiment; they do not certify a palette as correct.