In the Munsell color system, and in painting generally, the word for color intensity is “chroma.” Another word that means almost the same thing is “saturation.” Saturation is commonly used in computer graphics to describe color intensity. They don’t have quite the same meaning.
Saturation runs on the same scale (often 1–100) regardless of the hue or the value. So the most intense blue, no matter what, is a saturation of 100. The most intense yellow, similarly, has a saturation of 100. That’s very convenient for people who design software interfaces. And if you don’t really understand color, it makes perfect sense. What it doesn’t do is model color accurately.
In reality, the most intense yellow is far more intense than the most intense blue. And the highest possible intensity varies depending on how light or dark the color is. The most intense yellows are found at a relatively light value. The most intense blues occur at somewhat darker values. At very high and very low values, maximum chroma goes way down. But you can always set the saturation to 100 to get the most intense color at the hue and value you have currently selected.
Chroma runs on a different scale. In Munsell, the most intense chroma for yellow runs up to about 18. The most intense blue is more like 12. (I’m doing this from memory, so these numbers might be a little off.) Those numbers change with hue and value, so that the Munsell color space is an irregular, lumpy cylinder. That is a much more accurate depiction of human color vision than the saturation model. It helps to think about color in this way when you’re trying to understand actual color relationships.
Update and Correction
28 February 2008:
David Briggs, in comments to this post, writes:
To clarify, David, my objection is that you seem to be judging saturation as if it was meant to be a measure of colour intensity. It isn’t, it’s a measure of colour purity. The term for intensity of colour of light is “colorfulness”, the product of the saturation of a light stimulus and its brightness.
Many colours reflect light of high saturation, but only those that reflect light of high colorfulness (high saturation AND BRIGHTNESS) have high chroma. RGB colours that emit maximum saturation red light range from very low chroma (nearly black) through moderate chroma (dark ruby reds) to very high chroma (bright red or R 255). The importance of this concept of saturation for painters comes from the fact that when a coloured surface turns from shade into light, the colours of the light reflected from it tend to follow a uniform saturation series such as this.
And darn it, he’s right. Thanks, David. As this Wikipedia entry on colorfulness puts it:
colorfulness is the perceived difference between the color of some stimulus and gray, chroma is the colorfulness of a stimulus relative to the brightness of a stimulus that appears white under similar viewing conditions, and saturation is the colorfulness of a stimulus relative to its own brightness.
I, with my obviously very small brain, still find it most useful to think about color using the characteristics of value, hue, and chroma. But saturation does not mean what I said it does. I apologize for any confusion.