Hugs and welcome. Try not to think too much about how you should conduct the activity, you'll find Children with the freedom to explore will enjoy and guide the activity themselves, including the progression to painted hands & print taking :) - all good for evidencing engaging material and promoting self direction.
What colour paint are you going to have? if this can be mixed to obtain different shades of brown/green/orange - red & yellow have some ready to offer. I'm not sure what age group you're with but maybe ask if they'd like to make a pot of paint lighter or darker - maybe have some sticks instead of brushes ready to stir the paint.
The leaf shade/pigment is to do with the chlorophyll in leaves, you could talk about how the leaves grow older through the seasons/year, changing ... until late Autumn when they fall from the tree, make food/compost for insects & worms on the ground and how some trees keep their leaves - Christmas trees and others lose them ... Horse chestnut/conker tree -
Autumn leaves from wikipedia
I hope you have a great ob and enjoy it
let us know how it goes.
xx