I have s4 EE's--each lays a different color- so I can tell whose egg it is (lucky for me, cuz I like to keep track!).
I get blue green, green, tan (with brown splotches), and red.
The shade of the color changes from light to deep pretty regularly and more often as they get older. The green ones change the least, but I think that's because with EE (and americaunas) that the green color is the base color (notice it's green even inside the shell-- even the tan one is green inside)
The main variations I get are the light/deep-ness of the color and the amount of splotches if they have it. I'm sure diet plays a role and since mine are freerange, the variation is great.
edited to say: All mine started with their color from the first day on; also they've never "changed" color.
Sandra