Luminance refers generically to any measure of a color's intensity lightness, or brightness.
There's more than one way to measure luminance, and these different methods result in color spaces with different qualities. Make It Colorful includes three measures: value, lightness, and luma.
Value is the simplest measure of luminance included in Make It Colorful, and is simply equal to the maximum of the red, green, and blue components of the color. It is equal to 1 for any color on the hue spectrum, and remains at 1 for any such color that is blended with any amount of white. It equals 0 when all components are 0, that is, when the color is black.
This behavior can seem unintuitive in certain cases. For example, it is odd that red and white have an equal value of 1.
Lightness is equal to the average of the minimum and maximum of the red, green, and blue components of the color. Thus, in contrast to value, lightness equals 1 only when all components are 1, that is, when the color is white. But like value, it equals 0 only when all components are 0, that is, when the color is black.
This behavior provides a more sensible measure of luminance than value, allowing for white to be represented as lighter than solid colors such as red.
Luma is a more complex measure of luminance, designed to effectively indicate apparent luminance, according to the biological configuration of the human eye. The result is that the red, green, and blue components of a color are not weighted equally when determining luma. Instead of all three receiving a weight of 1/3, red receives a weight of 30%, green a weight of 59%, and blue 11%. This is because the human eye perceives green as much more luminant than the others, and blue as much less luminant.
This weighting provides a more balanced behavior when comparing colors of different hues. Whereas value and lightness would both measure yellow and blue as having equal luminance, luma considers yellow (luma of 0.89) to be significantly more luminant than blue (luma of 0.11). This property is particularly valuable when interpolating between two colors with different hues, to avoid banding or jumps in apparent luminance.