Searched: Heterochromia, different coloured eyes, chimera, mosaic, birthmarks and combinations of these, read the article on Wikipedia and a great lot of sites I found on the subject on Google; I'll be very ashamed if I missed it
Circumstances: I'm developing a character with a dark spot or discolouration of skin around one of her eyes as this is relevant to the story. I'd like her to have a very dark brown or black iris on this eye too, but couldn't figure out how this would have happened. The character is a middle-aged, Caucasian female, her other eye would be anything from grey to blue or light brown
Question: As far as I've gathered skin colour and eye colour seem to be determined by different genes, and thus a birthmark in the eye area wouldn't affect the colour of the iris, and vice versa. Or am I mistaking? Is there a way from nature's side both eye and skin could be affected by pigment? Or could any damage done during any stage of life give the same effect?