Calico cats are cats that have 3 fur colors and are always female or males two X chromosomes.
I've seen many cats that will have 2 fur colors (one of them is usually white) and another, third, color which is a darker shade of one of the other colors. As exampled in the pictures below (taken from the internet). Are those considered Calico cats?
pic of cat 1 http://i3.mirror.co.uk/incoming/article1381837.ece/ALTERNATES/s615/%C2%A3%C2%A3%C2%A3%20Larry%20the%20Downing%20street%20cat%20fights%20with%20the%20George%20Osborne%20cat%20Freya-1381837