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rgb-under color_1 color ...procedure
rgb8-under color_1 color ...procedure
hsl-under color_1 color ...procedure

Composite zero or more colors over color_1, similar to "normal" layer blend mode in image editing software. Same as rgb-over etc. except the order is reversed: the first color is on bottom and each later color is higher up.

The result type and color space used depends on the procedure. The result's alpha will be based on the inputs.