Direct color variations alter the color index inside of the variation calculation. The final color index used will be a blend of the usual color index calculation along with the one calculated inside of the variation. The blending is specified by the direct color parameter. A value of 0 uses only the the normally computed color index, and none of the value computed inside of the DC variation. A value of 1 does the inverse. The default value is 1. Note that this field has no effect on non-DC variations.
DC variations usually have a name which starts with “dc_”, however some variations without this prefix also alter the color coordinate.
You can see which variations use assign, rather than sum by observing whether it has a green square in the Type column of the Variations Tree.
Alternatively, you can find out which ones support DC via the command line by running:
which will return this list: