Color & LightRevised
Tone
The relative lightness or darkness of a color or area, independent of its hue.
In practice
Tone is how light or dark an area is, separate from its hue. Designers check it constantly because it carries most of what we read as form and contrast. A poster with a red headline on a green background may look lively in color but vanish in grayscale if both colors share the same tone. Viewing a design in grayscale quickly shows whether its tonal structure works.
Often confused with Saturation
Tone measures lightness; saturation measures how vivid a hue is, so two colors can share a tone while one looks bright and the other dull.
Try this
Convert a colorful poster or screenshot to grayscale and check whether the most important element still stands out.