At what age does kitties change their eye color?

I have four kitties..one black, two gray with stripes and one that looks like siamese. All had very blue eyes then a couple of weeks later one had very green eyes that now are green-yellowish...i was like "wow" that was a big change...now the black one has greener eyes too...the blue is almost lost, and the other gray has hazel eyes.all but the "siamese" one changed their color from blue to green to a mixe of green and yellow. What can you tell me about that? I'm glad at least one kept the very blue eyes. :)