Applies a colour to the picture, three different ways, and the difference between them is what survives.
overlay simply replaces the picture with the colour, so alpha becomes a plain opacity control and at full strength nothing of the original is left.
multiply keeps the tonal structure. Anything multiplied by a small number stays small, so the dark areas stay dark and only the bright ones take the colour. That is how a coloured gel over a light behaves, and it is usually the one you want.
recolor is the most selective. It keeps each pixel's brightness exactly and replaces only its hue, so the shading is untouched and just the colour changes. Worth knowing: the saturation it carries through comes from the green channel rather than the pixel's actual saturation, which is a quirk of this implementation and explains the occasional over-vivid result.