@Thrond  
The show foals have always followed a formula. And I’d argue even in outlier cases it’s due to a grandparent we’ve not seen before. Pony colors are genetic, they just don’t work the way most artists think they do when they just blend them together. Seeing a great drawing of 10 offsrping characters with great designs is often ruined by the colorations all being very muddy. And as they have their own children, all the colors will get closer and closer to just being brown and grey. And no-one wants that.
 
More proof of there being a system to it:  
>>1797311t (merged)

@Plko2  
Granny Pie*

@Thrond  
Overall the show actually follows those rules aside from some real outliers like Cake Twins or Pinkie. Just look at Fluttershy and Zephyr compared to their parents.

@Thrond  
Well considering a light brown pony and a light grey pony had a hot pink pony that is the twin of a grey pony it seems color can be arbitrary. Or it could skip a gen or two. Maybe one of their parents was pink. Nana Pinkie maybe.

I really don’t feel offspring characters ought to follow any rules at all regarding colouration. It all seems sorta arbitrary in the show itself. Just be creative, I say.