I've been having trouble with the SkinPreserver, and wanted to make potions that change the sim's skintone instead of just applying a temporary costume (even if I get the skin preserver to work, I'd still like to know how to do this. I've already managed to create a skintone with a custom mesh:D)

If someone can tell me how to do this, I'll make a fancy mesh for the new potion.

You cannot do that. In fact, even EA/Maxis cannot do that. The way things like plantsim and vampire skin work is by kludging every behavior that can possibly affect appearance (i.e. dozens of behaviors).

Would there be a way to make a controller item that would change it (for a specific sim) every time the lot loads?

Lot loading isn't the issue. The issue is any time a sim changes appearance (clothing, accessories, skin condition), the entire sim changes. You'd have to change how all those routines work, and there are dozens of those. Not impossible, but very complicated and you'd need several versions depending on what EPs the player has.

But would you it be possible to set an item up so that a sim could "equip" it (like a helmet or ring), and as long as it's "equipped", they have the skintone?

I'm working on a few items that temporarily change a sim's appearance, and possibly allow them to use custom items, I'm just not up to par with BHAV's yet.

Actually, that is not how it works, and such a thing is, in fact, possible, but would require a redesign of most skins. See Morague's "Overlay" tutorials for some starting points. I have had positive results adding belly hair to certain specific sims without actually genetically altering them. It is similarly possible to tint existing skins to produce minor color variance without remaking an entire skin or introducing the screwed-up genetics of custom skins.

I'm looking to make changes like SimPE does, without having to make a "sacarificial sim", or go into SimPE. I know that Vamp/Zombie/Lycan/Plant have to be reversable (some players wouldn't be too keen on their favorite sims becoming perma-vamps), but this is a different type of mod I'm working on. It also has a change of wants/fears as well as adjustments to skill learning and motive decay rates.

When this is done, if at all, it should be very interesting