This set of 2 cutters are what I use when mixing new colours. 
 
Use either the 6 or 8 part pizza cutter to quickly and efficiently cut a piece of clay into 6 or 8 eaqual parts, ready to be mixed into your new colour, in a repeatable recipe. Noting down how many parts of which colour you used will help you recreate the same results later on in any quantity of clay that you might need, without waste.
 
The discs are 5cm in diameter, meaning that the parts of the pizza are small enough that your experiment will be easy to do and the results clear, without being so big that if the colour mix is not working out, you haven't wasted a whole lot of clay in the process.
 
Check out this video to see me use the 8 part cutter.
https://www.youtube.com/shorts/H6--HFdZdwQ