In citrus fruits sometimes ripe fruits will turn orange and then turn green again (either all over or in patches), it’s called regreening. It’s usually due to either chilly night temperatures or high UV exposure, so the same tree might produce completely orange or orange and green or completely green ripe fruit all in the same year.
Kumquats are taxonomically weird and may or may not be citruses, so I’m not sure that’s what’s happening in the OP, but I would guess it probably is.