This command lets you change the the colour of any dyeable item as many times as you want, to whatever colour you want! Dyeable items include pouches, leather armour, party hats, and more.
To do this, use /itemdye <hexCode> while holding the item.
A hex code is a hashtag followed by 6 characters, e.g. #5555FF is the hex code for blue. There are many free colour pickers available on the internet, which let you visually choose a colour and then copy its hex code. You can just Google "colour picker" and Google's in-built colour picker will appear at the top of search results. There are over 16 million unique colours you can choose under the hexadecimal system!