Update on Person Cozy: it'll be a while.
I didn't swatch. Rather, I used my hot water bottle cozy as a swatch to figure out gauge and how the yarn would behave after blocking. I figured this was perfectly reasonable - I was using the same yarn, same size needles, same stitch pattern - the only difference was the colorway. And oh, what a difference it made!
Berroco Peruvia in "sea turtle" is soft, squishy, warm, luscious with just enough of a halo to make it cozy. In black, however, it is scratchy, straw-like, less elastic, itchy and hairy. "Halo" is not the word - it's hairy. I noticed the difference early on while knitting - the yarn wasn't as soft, it was hairier, it was splittier than when I'd knit it in "sea turtle." I figured it would all even out after blocking. I was a fool.
The lesson I have learned, and I hope you'll all learn from my mistake, is to SWATCH!!! Swatch even when you don't think you need to. Switching to a new yarn of the same weight? SWATCH!!! Switching to a new colorway that feels slightly different? You're not crazy - SWATCH!!! I feel like I've flushed an itchy, hairy sweater's worth of yarn down the toilet. Ugh.
But in reality, I haven't. I have a prototype, I tried it on. I now know that the basic design works the way I wanted it to. I now know that if I raise the neckline and shorten the shoulder straps, it will look perfect. I now know exactly how much negative ease I want (it fits great width-wise, but the black yarn had less vertical stretch than the green yarn). I will find suitable yarn, I will swatch, I will knit a perfect and glorious Person Cozy, and it will be good. Someday. When I can afford the yarn. Like the phoenix, Person Cozy will rise again!