Recently, I found a skin set that would work wonderfully with my idea for a colourful legacy. They were custom, but I managed to geneticize them. Everything was fine and I was excited to start! But just to make sure that everything was working properly, I rolled the pacifier in CAS. The first Sim to spawn was a default S2 Sim while parents were blue and pink. I completely forgot about the defaults and no matter what I did, I couldn't get rid of the defaults mingling in with my unusually-coloured Sims. I tried to learn how to default my own skins, but it confused me and left me discouraged. I really want my rainbow Sims, but the default skins are ruining that for me... What do I do?

Default skins you simply remove. They replace the Maxis skins.

The thing is, I don't have any default replacements. They ARE the Maxis skins. Maybe I'll just try to replace the Maxis skins with the colourful ones again...

You need to pick out four skins you want as defaults to start with. There is an easy method of creating defaults which is that you use an existing (working) default from somewhere and just replace the textures in SimPE with your preferred skin textures (which you can extract either with SimPE or Bodyshop). I can give you more details if you like (or maybe find the tutorial that describes this). Be warned though replacing complete skin textures for a skintones is a lot of textures because there are images for heads, scalps and bodies for all ages (baby, toddler, child, teen, adult, elder), both genders and the three body states (fat, thin, fit). It's a bit of a fiddle to do manually but it's an easier option if you're not good with SimPE than creating a default skintone from scratch.