Sift the powdered sugar and set aside in a large bowl. Place vegetable shortening in a stand mixer bowl.
Heat marshmallows in the microwave on high (or on stove top) in in 30 second increments until melted. Stir with spoon. Marshmallows should be ooey-gooey and ready to be added to the mixer bowl.
Pour the water on top of the marshmallows to get them to release from the sides of the bowl. Pour into bowl with vegetable shortening.
Turn stand mixer on lowest setting with dough hook attachment. Add in half of the sifted powdered sugar, a measuring cup at a time, and let mix for 2 minutes (it will be really rough looking at first). Don't stop mixing until it's sticking to the sides of the bowl and looks smooth. Add in another cup of powdered sugar. Pull fondant off the dough hook attachment by putting vegetable shortening on your fingers and pulling it off the hook. Take the soft mixture out of the bowl and put it into the large bowl with the rest of the powdered sugar.
Add color as desired. For dark colors especially, add the color during the mixing process or you could get a sticky mess.
Warm the Wilton fondant in the microwave for 40 seconds and add to the large bowl with the powdered sugar and marshmallow mix. Knead until powdered sugar, marshmallows and Wilton fondant is mostly incorporated. You may not use all of the powdered sugar depending on your climate and that is completely fine.
Pull fondant like taffy until it is stretchy and smooth. If there are still rough spots or it's tearing, put everything back in the microwave for 30-40 seconds to make it really hot and pull like taffy with shortening on your hands until it pulls without breaking.
To use again, reheat and knead the fondant well until stretchy before each use.
For coloring fondant with dark colors, use 1 Tbsp warm water and 1 Tbsp food color gel.Store in a zip-lock bag at room temperature. Fondant will keep for months in a zip-lock bag.