Silicone Brushes 
Silicone Brushes – Color Shapers BLACK are excellent tools to sculpt and add incredible effects when modeling and sculpting with different putties, clays, and fimo. They are somewhere between a brush, a palette knife, a modeling tool, and your fingers. Easy to clean and long-lasting. Non-absorbent.

This is the firm version of the black silicone brushes. They would be twice as hard as normal white brushes, and half than extra firm.

This set comes with 5 different brushes with total length with 16.3 cm length by 3 mm width - SIZE #03

This set comes with 5 different brushes with total length 17 cm different length by 5 mm width - SIZE #5

This set comes with 5 different brushes with total length 20.2cm length by 8mm width - SIZE #8

Each brush in the offered sizes has different uses depending on the putty we want to use. Nevertheless, Green Stuff World recommends using White tip brushes for soft putties, and SIZE #2 for harder materials.
This is a question of taste and preferences, but in the ending as your sculpting skills get better you will finally end up having 1 set of each.