Value contrast (tonal contrast)
The lightness gap between elements, hue aside. If the design still reads as a grayscale screenshot, the contrast is real.
See it
What it is
Value is how light or dark a color is once you forget its hue. Value contrast is the gap in that lightness between two things: text and its background, a card and the page, a logo and the photo behind it. It is the contrast your eye actually uses to separate shapes, which is why painters do a grayscale value study (and why the Japanese notan tradition works in pure black and white) before touching color at all.
Reach for it whenever a layout feels flat, busy, or oddly tiring despite having plenty of color. Screenshot it, desaturate it, and look again: if the hierarchy disappears in grayscale, the color was doing the work that lightness should have been doing. Squinting does the same job in two seconds.
Gotcha: hue difference is not contrast. Pure red on pure green is a shouting match with almost zero value contrast, so it vibrates, blurs at small sizes, and vanishes for a red-green colorblind reader. WCAG contrast ratios are essentially a value measurement, which is why picking a prettier hue never fixes a failing one.
Ask AI for it
Audit this design for value contrast: convert the palette to grayscale lightness values, list any two adjacent elements whose lightness differs by less than 25%, and propose fixes that change lightness only (keep every hue and chroma where it is). Give me the before and after lightness numbers plus the resulting text contrast ratios.