Darken and Lighten
Darken:
compares luminance values of each layer
uses only the darker values
RGB channels compared separately, hue changes
Shade:
legacy version of Darken, functionally identical
Darker Color:
Like Darken, but compares all 3 channels together
Lighten:
compares luminance values of each layer
uses only the brighter values
RGB channels compared separately, hue changes
Lighter Color:
Like Lighten, but compares all 3 channels together