I'm considering making a number of Pro Designs for clothes but I have a few questions first.

1. Is there a way to stop colors from changing when I switch palettes? Whenever I switch from one pallet to another, all of the colors change to whatever their corresponding color on the new pallet is and that is such an annoyance. Is there a way for me to prevent this?

2. If I can't prevent the color changes, can I make my own pallettes with the colors that I need?

3. How many custom palettes can I have and can two palettes share colors? A lot of my designs will use the color black so I do need to be able to have it available in multiple palettes for this reason.

Thanks in advance!

Nope.

You can't make your own pallets in City Folk unfortunately. Each time you change the pallet ## you are going to get other colors. That's not awful if you are doing paths, and want to see different color combinations, but not so much for clothing.
The only game where you can make your own pallet is in Animal Crossing New Leaf.

That is really annoying. I can get not having custom palettes but who thought that having the colors all change whenever you switch palettes was a good design choice? It just makes everything more irritating.

Unfortunately, every game from City Folk back had this problem. New Leaf done good with their new palette options...maybe in the next game they'll bring back the button controls for designs too.