I want to start making some Berry sims in my games with brightly-coloured skin and hair colours, but the problem is that I don't want random townies being generated with those skintones. Is there any way to make it so the skintones are disabled for random? I don't want random sims using the skintones, just the ones I make....

I'm not an expert in CAS but I believe everything CAS is using the same flags for that. Try opening them and check? Here is a tutorial on how to edit the flags; more detail on the random stuff: http://modthesims.info/showthread.php?t=560284

Skin TONEs don't have any flags that enable for random. However, they use the same property tag/value pairs as CASPs, and if they have no Archetype tags they won't be chosen randomly unless there are no other TONEs with Archetype tags. (Human EA skintones all have Archetypes; alien EA skintones don't.) In TONEs the Occult/Occult Allowed for Random tag determines whether the skintone can be used at all by humans/aliens/whatever, so it has a different function than in CASPs.

if you're using s4pe I can upload a dll that will let you see and modify the tag/value pairs. Or if you can wait a couple of days, I'm currently finishing up a TS4 Skininator skintone cloning/editing tool that should make it much easier to do this.