This might sound silly but I struggle to get my head around even some of the more basic parts of object creation and tools like TSRW. I tried to search for this issue but maybe I'm not looking in the right places.

If I wanted to share a new, standalone recolour (i.e. one that doesn't overwrite the original presets) that I've made of another person's non-CASTable creation, how should I package it so that it will show up in somebody else's game as a recolour of that object but only if they already have that object first?

You need to clone the object using s3oc, but unchecking the 'rename/renumber internally' checkbox before proceeding to clone. By this method you are now making an override package file, then edit it using Texture Tweaker to add the recolorable preset you wanted to add.

Also, your finished package file should go to the overrides folder so that the changes would take effect..

This shouldn't be necessary for CC, unless it's missing data in its package for some reason this wouldn't change it in any way. Just using a copy of the original package and renaming it would be sufficient.

The important part would be making sure to delete everything in the package except the OBJD resource and any new textures that are part of the preset after editing with TT3, otherwise there would be nothing preventing it from working by itself.

Thank you both!