Bottled Inks not only offer you a rainbow of colours to choose from, they're considered to be more economical per fill and offer a more sustainable solution than their cartridge refill counterpart.
If you need an ink with special properties, for example, being waterproof, uv & forge resistant, archival etc... Noodler's hand crafted inks offer a variety of options.
If you're looking to try a new colour without the size or price investment, Colorverse Mini Bottles are a great option at only 5ml per bottle.
Other fan favourites include Lamy, Papier Plume & Pilot's Iroshizuku.