When I load up my small piping bag (different technique for large bag)
I have a similar technique but different. I will explain:-
Get a tall plastic cup that is narrow on the top. Place your piping bag inside it with the top of the bag over the rim of the cup. It has to be a tall cup because in the next step
You get a Freezer Bag (a light weight plastic) and pull one of the two corners into a point
Push the pointy end into the hole in the end of your piping bag in the tall cup until it pokes out an inch (so it doesn't accidentally go back in the bag)
Open the end of the freezer bag over the rim of the cup
Now you have a firm edge (the top rim of the cup) to scrape the palette knife edge onto with your buttercream or royal icing until you have a fist full inside
twist the plastic one turn at the open end (not the nozzle end or you will block it)
secure the end of the outer bag with an elastic band. Cut the end of the plastic off poking out the tip leaving 1cm which you poke into the piping tip. Secure your nozzle and squeeze to remove the air bubble. Now pipe.
As you use the icing the elastic can be rolled down the bag to create a little pressure inside the bag
Put a small wet sponge inside the bottom of the tall plastic cup and when you are not piping place the piping bags nozzle tip down inside the cup. This keeps the nozzle tip moist and stops it blocking as it should not dry in the sponge. (No need to unblock with pins which damages the tip)
When you have finished with the icing in the bag and you want to recolour it for another project, pull the bag out and squeeze the icing or buttercream onto your granite/glass/marble top. Palette the colour in on the flat surface and refill again.
You should never have to wash your piping bag or get sticky hands.
NB If you keep the icing minus the piping bag in a plastic tub in the fridge overnight, the end may get crusty so cut it off with scissors before squeezing out the contents in the middle of the bag to rework it.
NB This is how it was done when I was decorating easter eggs in a production process one after the other for a chocolate factory. I used the same technique in my shop.
Wedding Cake Enchantress