I want to try some different face mods, but I am having a difficult time changing lip colours. Varies from mod to mod, I can change colour in Bella's mod just fine, but not in CoverWomen or Seductive Lips.

I know that I must be doing something wrong, as I see no complaints in the comments for these mods.

If you mean lip color in the CK, then the lip tint-masks from those mods need to be re-saved as R8G8B8 with no mip maps, then re-sized to 256x256.
Paint.net works well for this.

I am fine with changing colours in the CK, I just have to temporarily re-name the appropriate lipmask, change the color, save. exit then re-name the lipmask back. But with some mods this technique doesn't work. I don't want to have to make all my lipmasks low-res, that would be defeating the purpose of using hi-res mods, yes?

When you CTRL-F4 export the FaceGen facetint, it's only 256x256 anyway. I don't know what size the player's facegen facetint is in the save game. I would imagine it's the same. I think the hi-rez facetints are only visible during character creation.
The resized modded tintmasks still look better than vanilla

So is there any point in having the larger resolution files? Does this apply only to the lips? Thanks!