Colorfulness is a non-technical term for the measure of how intense a specific color looks. It is related to saturation and chroma (Munsell). Colorfulness can simply be said to be the degree of difference between a color and gray. But what gray? Saturation on the other hand takes into account that all colors are not equally bright when fully saturated: For example blue is darker than yellow and red. Saturation thus means the colorfulness of a color in relation to its own inherent brightness. Laymen will find that the terms colorfulness, saturation and chroma are used synonymously. The more colorful a color is, the more vivid and intense it seems, while less colorful colors appear muted and closer to gray. When all color has been removed and saturation is 0, the image is called a gray scale image. There is an overlap between lightness and saturation in that a more saturated color appears lighter.
Lightness, also called value (Munsell) or tone, is defined as a colors placement on a brightness scale ranging from black to white. In the Munsell color space this scale is divided into ten equidistant steps. The term “lightness” has been incorporated into the HSL (Hue, Saturation, Lightness) and Lab color spaces. The term “value” is also used in the HSV (Huse, Saturation, Value) color space, but used differently than in Munsell. In HSV value ranges from black to not white, but to the fully saturated color. Paints can be made lighter or darker by adding white or black, but that also reduces saturation. “Tone” is an obsolete term that stems from darkroom photography to denote the lightness of a specific area of the print. Yet “tone” is still used in art where light and dark “tones” are built up with charcoal or similar drawing medium
In digital phtography lightness can be calculated simply as (r+g+b)/3. However, that does not take into account that green is brighter than red and blue darker than red. The relative brightness of the color channels is taken into consideration in the IUV color space, that calculates lightness like this: i=(76*r+150*g+29*b)/256. It approximately says that green is twice as bright as red and red is 2½ times brighter than blue.
In painting the term “tone” denotes an intermediate between gray and pure color. A mixture of pure color and white is called a “tint” while a mixture of pure color and black is called a “shade”. In reality tint and tone are not as simple a it sounds, because though they do not have color in themselves, when white or black are mixed with a color, the color changes hue. In other words, black in mixtures behaves like a blueish color. For example you get a darker greenish yellow if you mix yellow and black, not just as darker yellow. Similarly will an addition of white make a color appear colder.
In software you can digitally create tints and shades by converting from the RGB colorspace to IUV or Lab colorspace and alter the L (lightness) channel as described above. This will not alter the hue of the color as when you mix pigments with black or white.
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