Place the marshmallows and butter in a very large, microwave safe mixing bowl. Microwave in 15-second blasts, stirring between each burst until melted and smooth. Be careful, it can get hot.
Add the orange food colouring and mix until the colour is evenly distributed. If your mix starts to set, you can pop it back in the microwave for another short burst.
Add the rice krispies in two batches, mixing until evenly coated.
Lightly oil your hands and take small handfuls of the mixture (25g portions / a little under an ounce). Place them on a lightly greased surface. You should get 20 in all.
Use your hands to roll a portion into a ball, squeezing to compact. If you find it's too sticky to work with, you can use an oiled piece of baking paper to help you. Repeat until you have 20 neat balls.
Push a piece of pretzel stick into the top of each pumpkin to make the stem. Leave them to set and lose their stickiness while you make the royal icing.
Make the royal icing
Put the icing sugar, cream of tartar and egg whites in a bowl. Whisk until well combined (it should be very thick).
Add the green food colouring, stir and add more as necessary until you have a leaf-green shade.
Transfer to a piping bag and snip 1-2mm (1/32 - 1/16 inch) off the end of the bag.
Pipe leaves and tendrils onto the pumpkins and leave them to set.