Described in the book as:
“This short sock has a heel worked with a mosaic (two-color slip stitch) pattern, while the foot features an articulated toe for easy wearing with your flip-flops.”

Yardage given is for 3 full balls of suggested yarn (2 main color MC, 1 contrast color CC). You won’t need all of the yardage for the CC and probably not all of the 2nd skein of the MC.