How do I change color for a certain design (uploaded as vector) so that it changes for all products with that design? Right now I have to click on every single product and do the color changes.
It’s not possible to do this by a one click command. 
Indeed as specified here under Vector Graphics: https://help.spreadshirt.com/hc/en-us/articles/360008958033-Faster-Product-Creation-with-Templates