How light or dark a tone or value appears also depends on what other tones are near it. The two vertical bands of tone in the image above are of a consistent tone, yet seem to get darker or lighter depending on how light or dark the background is.
This effect is most noticeable with mid-tones, than with very light or very dark tones. And, of course, it applies regardless of the actual color or hue. Take a look at another example, in brown tones if you need convincing.
So what use is it knowing about tone being relative to the tones around it? For starters, it shows that if you want a light tone, you shouldn't just reach for white (or add lots of white to a color). If the overall painting is dark, a mid-tone may be light enough for the effect you're after, while an extremely light tone may be too harsh.
The same, of course, applies to darks. If you need a shadow, for example, judge how dark it wants to be by the tones that you've already got in the painting. Don't automatically go for an extreme dark; the contrast may be too great for the overall balance of the photo.
Think of tone as an element in a painting's composition. The tonal contrast or range in a painting, and how these lights and darks are arranged, needs to be considered when you're planning a painting (or trying to figure out why it isn't working). And a painting doesn't necessarily need a wide tonal range to be successful; a limited range of tones can be very powerful if you use relative tone effectively. As with the number of colors you use in a painting, less often produces a better result.
