You essentially have 3 stages:
1. Belt to pink pulley
2: pink gear to green gear
3: Planetary stage
I'm going to assume your motor is stationary, and so is the purple ring gear.
Have to assume there's a planetary carrier, not shown, that is your output.
Pulley reduction will be the ratio of the diameters, moving in the same direction.
2nd stage reduction will be the ratio of the diameters, reversing direction
3rd stage is slightly tricky because of the stepped planets. Check out the beginning of this article for the method called superposition to derive the gear ratio:
https://www.engineeringclicks.com/planetary-gears/
The reduction, I believe, is (1+(RingDiameter*PlanetSmallDiameter)/(SunDiameter*PlanetLargeDiameter) to 1)
Output in the same direction as the sun.
Multiply your various stages by each other and keep track of direction for total reduction and direction (one reversal, so opposite to motor is direction).
I assume that's confusing, so let me know where you'd like clarification
