Freckles are considered a free marking in Isle of Sagittari. You can give them to your centaur freely.

Freckles can only be displayed on the centaurs upper torso. They must be sharp or very slightly blurred dots, not big enough to be too similar with spots

Freckles can be darker or lighter than the basecolor, and must follow the basecolors saturation. 

You are free to spread them around the upper torso as you wish, or create clusters of them.