This post is old so maybe you’ve learned this by now, but color changes are only permanent in the adult stage. If you do the color change as a toddler or teenager it will revert to the original color in 24 hours or when it ages to the next stage. If you change your teen’s color to orange, it will not become an orange adult.
I’m pretty sure color changes always affect both twins.
I’m not sure if I remember this correctly, but I think the two color changes to unlock the salon location have to be in the same stage. That’s easiest as an adult since you have basically unlimited time but the color changes would be permanent. If you don’t want it to affect your adult’s color, you have to do the two changes as a toddler or teen which means you have to buy the foods for the color changes and basically only feed it those foods during that stage.
Hope this helps someone (: