This command fills each channel of the selected images with the corresponding value taken from the argument list.
The first value on the argument list, c0, initializes channel zero of the selected images; the second value initializes channel one, and so forth.
The number of items on the argument list need not match the number of channels in the selected images. Excess arguments are ignored. If there are an insufficient number of arguments, the unmatched channels are set to zero. However, the argument list must have at least one value to be assigned to channel zero.
This command has a simple remit: fill an image with a solid color. This is the aim that many people have when they turn to -fill. Indeed, this command is largely a wrapper around that command, inducing it to iterate over the channels of the target images, successively filling those with the corresponding value on tap from the argument list.
Typically, the argument list has the same number of values as there are channels in the selected image, but -fill_color does not require that. With respect to a particular image in the selection, if the list of values in the argument list are in excess, the excess is disregarded. If there are an insufficient number of values on the argument list, the excess channels are set to zero.
Thangka Paint Pots Mrs. Cardboardbird, Wikimedia Commons