I think sometimes its commonly thought that the only way to add a colour to a pot is through glaze and coloured slips take a back seat to the dazzling colours of glazes.
A coloured slip can be applied to any leather hard pot it’s actually made from clay itself which has a smooth texture from being mixed with water. Often this slip or slurry is excess clay from during the throwing process mixed with a coloured pigment. Sometimes more complex slips can be reactive such as a rutile which when glaze fired brings out a mixture of colours and textures.
Due to their simplicity they’re usually more predictable in the firing process and more stable than glazes which you need a recipe to concoct. To seal a pot with coloured slip on it all you need is a transparent glaze which is a basic glaze with no colouring in it to reveal the slip underneath.
I’ve sometimes had mixed experiences with coloured slips; Because you have to brush them on to the pot sometimes you end up with a streaky or uneven texture, although that is part of the charm!
The main problem though is getting the consistency right and balancing the amount of pigment, some colours like blue can look very washed out rather than giving a bright cobalt colour. If you can’t tell I’ve recently been using slips again to create a set of dishes, working with them again has changed my mind about slips so maybe I should use them more often!