What tone means in a painting context is simple. It's how light or dark a color is, rather than what the actual color or hue is. Yet implementing tone in a painting is often bothersome to artists because we get distracted by the strong appeal of color.
Every color can produce a variety of tones; how light or dark these are depends on the color. It's important to realize that tones are relative, that how dark or light they seem depends on what's going on around them. A tone that's obviously light in one context may seem darker in another if it's surrounded by even lighter tones.
The number or range of tones that can be produced also varies. Lighter hues (such as yellows) will produce a smaller range of tones than darker ones (such as blacks).
Why is tone important? Here's what that master of color Henri Matisse had to say (in his A Painter's Notes, 1908): "When I have found the relationship of all the tones the result must be a living harmony of all the tones, a harmony not unlike that of a musical composition."
In other words, if a painting is going to be successful, you must get your tones right, otherwise it's just going to be visual noise. The first step to doing this is to remove color from the equation, to create a range of tone using only black.
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