I'm making some recols here that I'd like to upload later. I could only get it as a default replacement at the time though... so if I make recolours of that in body shop, will those work for people who have the custom version later, or are they getting weird looking versions of whatever hair they have for the one I got that default for?

Not understanding the question. If you have made a default replacement hair then it will be a default replacement hair. If you have made a custom non-default then it will be a custom non-default. It will look like on everyone else's computer what it looks like on your computer if you are meaning the color.

Are you asking how to turn a default replacement into a non-default replacement?

No, what I'm asking is:

I replaced hair A with hair B. I recolour hair B in body shop, but I'm only having the default replacement in my game.

If I upload those recolours, and people download it who do have the mesh, but only as a custom version and not the default replacement that I clicked when creating the recolour in body shop - will it work in their game? Or will the game "think" it's a recolour of hair A that I had replaced with hair B, and not display it correctly?

Recolours are linked to the mesh via their GUID. If the default has a link to a mesh with a different GUID to the regular mesh, then no. Do defaults come with their own mesh? Do you have to have both the mesh for the default and the custom mesh in your game if you want to have both default replacement and custom versions of the hair in your game? In my experience, no, but that is going to depend on what the original maker of the mesh and default did.

In that case, it should work. Thanks.