A Difference in Colouration Between Two Structures Normally Alike in Colour

That is, heterochromia (usually referring to the eye or iris).

Which, as it happens (by no coincidence at all), is the name of the comic for which I may now present some concept art. But first, an explanation of the story:
Some time ago (between ten and eighteen years prior to the story), an alien race called the Satekae came to Earth seeking its natural resources. The resident humans of the time, however, naturally objected to Satekae taking control of their resources. The conflict elevated to an all-out war. The great majority of humans were killed, but a limited number were taken as workers by the Satekae. For some time after, the humans were to severely oppressed that much of their knowledge and culture were lost. Now, however, the shock of enslavement is wearing off, and with it, the instinct to become free grows. Panima heads plans for rebellion against the Satekae, her methods favoring violence and vengeance over preservation of what is left of mankind. When Tamino, a man from the "superpower" government (in a similar way as the United States of America are considered a "superpower") of the galaxy shows up, however, Pan finds herself switching tactics, instead trying to gain his favor so that he will convince others from his government to order humans' freedom. What a long-winded explanation. But, I haven't written anything in a while... Give me a break.