chroma is basically the relative colorness of the color relative to "white." This is how bright the color looks: add white, brighten the color. Ulitmately there is a fairly subtle difference between a true white and increasing the chroma of a color to the point where it looks white. (chorma can loosely be thought of as "lightness."
saturation is the relative colorness when compared to itself: This is how intense the color is. Desaturated colors look "greyed out."
colorfulness is the relative colorfulness when compared to grey. This is a description of value How light or dark the color is relative to a greyscale. Mid blue is the same value as mid grey, dark blue is the same value as dark grey.
Hue is the color that appears (blue, yellow, red ect.)
The interplay between these concepts is what produces the color that we see.
What Enion said is that in certain pigments, adding white will make the color look brighter, before it shifts. Think about red..there is a point at which adding white to red brightens the red. Go past that point and ou get pink, all the way to white. Highlighting red with white works because adding white also changes the value...a highter value (closer to white) makes higlights. It also takes the colorfulness out because pink and white look less red than red does.
Shading is mostly a matter of value. ou want to add value so you darken the color. Adding black is a very straightforward way of darkening the color...and it worlds in kind of the same way as white..you are still taking the chroma out of the color, just doing it with a darker value. (I think this is right-but I am fuzzier here). Again, as you add black to the red you decrease its chroma. until you get something that is a lot like black.
Desaturation can grey the color, but theoretically it should grey the color at the same value. In other words a mid blue desaturates to a mid grey. The color will get greyer but not darker or lighter (you can desaturate witha darker tone to get shadows..but at that point you are playing with both value and saturation. this is something like putting a black was over red to get shading.
Ok, they other way we can play with value is by adjusting hue. I like this method, but it is less realistic, and some painters don't like it for that reason. Highlighting by hue adjustment means that you change the basic color to a hue that is a lower or higher value. If you shade red by adding dark blue to it...you are shifting the hue to purple to create shadows (this is very commonly done with white-when its shaded light blue) or when you highlight red with orange and yellow. This is not a "natural" highlight, but looks really good-i think. In this case you are not adjusting the saturation of the color, or its chroma (well you can do this actually-but for this example I'm leaving it alone) but you are changing value. Purple is darker than red, orange is lighter than red, or so they appear-in fact they might all be the same chroma and saturation, its the appearance of value that is manipulated. Basically you are fooling the eye, in fact almost all painting is fooling the eye.
I had sort of intended this to be a simple translation of what enion said...and it ended up..well you know.
I'm not saying knowing this will make you paint as well as enion..i sure dont. But at least you can talk in very fancy terms about why you can't paint to enion.
