Time for my overanalyzing! :P
In the real world, there are animals who look the same in the early stages in life, male or female. And when they mature, you can easily tell the difference from Male and Female. Now with Kirlia's green legs, since it's not at the 'mature stage', it's hard to tell if its male or female; unless you look at it's....erm...parts.
When its reaches Garde stage, the appropiate leg color shows. But then again, I seen/drawn female Gardevoirs with green legs and males with white. So, I wouldn't let leg color be a foolproof way to tell gender.
In the other subject, I always thought Kirlia and Gardevoir had 2 layers of skin or somthing like it. Like Gardevoir's bottom skin layer would be white, and that 'top' it has on is the 2nd layer. That skirt it wears is actually flaps of skin used to 'protect' it's delicate legs from cold, ect. For Kirlia, the green skin of the legs would merdge with it's white skin under where it's skirt starts (again, flaps of skin...). But this can also be applied to Garde.
Ok, class dissmissed!