If you don’t have a converter you can reload the cartridges from bottled ink using a blunt end syringe. Many pen shops sell them. Example https://www.jetpens.com/JetPens-Ink-Syringe/pd/22182. Bottled ink opens up literally thousands of color shades. https://inkswatch.com

In case you don’t know, most fountain pen ink is NOT waterproof. There’s ink specifically formulated to be waterproof if that’s important. JetPens has a nice article about it: https://www.jetpens.com/blog/The-Best-Waterproof-Fountain-Pen-Inks/pt/829. I personally like Noodler’s archival / eternal inks.

Noodlers Bay State Blue is very pretty but will stain everything it touches, including your pen. It’s a wonderful ink but it’s best for either a dedicated pen or a non demonstrator so the stains don’t show.

Again, welcome to the wonderful world of fountain pens.