Okay, good to know I have hatched six green ones and I still need two more of them. I'll try to get the purples next.
Just quick edit to add quote from the release thread:
WE SOLVED THE CHART ON THE DISCORD CHAT!
A Siyat's color depends on their natural time left when hatched. If you incubated, add a day to the egg's timer to find the window they are in.
7d 0h - 4d 1h: green (this means you must incubate, earthquake, or try to force with kill for green)
4d 0h - 2d 1h: blue
2d 0h - 0d 0h: purple