So the way these slippers go together, you work flat for a while and then you work in the round (they’re built a lot like Quiescent if you played with those). And whenever you go from working flat to working in the round, you need to pay a bit of attention.
The first thing to watch is your gauge. Many people have a different gauge in one or the other, and you’ll want to do what you can to make it match. (How do you know if you do? Your swatch…it’s always your swatch!) That can mean using a different size needle, or even using the same size but a different material/shape. Like for me, I get a slightly tighter gauge on square needles than on round ones, and a slightly tighter gauge on wooden needles than on metal ones. And I knit a tiny bit tighter when I’m working in the round than when I’m working flat. So if it were a problem for a given project, I might switch from square needles to round ones or from wooden needles to metal ones when I move from flat to in the round to keep my gauge consistent.
The other thing to watch is your yarn, especially with multi color yarn. And this is more of a ‘gee, yarn is nifty’ than a ‘watch out for it and correct for it’ sort of thing. But take a look at how the colors on the yarn look in the top and bottom part of the slipper.
The top two thirds or so is was where I was knitting flat. The bottom third or so is where I was knitting in the round. It’s subtle (and shockingly hard to photograph), but look at how the colors sort of stack up in blocks on the top part and are more blended together on the bottom part (you can also see how much more even my knitting is when I’m working in the round, but hat will block out).
The color change is because of how the colors in the yarn stack up when you’re folding the yarn back and forth, row after row versus how they stack up when you’re spinning it in a spiral as you work round after round. And it’s totally not a problem here (and it’s not at all something wrong with the yarn…that’s just how yarn/knitting works). But it is something to be aware of so you’re not surprised by it (it can do funny things on like a sock heel or on a sweater when you divide for the arms).
Plus it gives us an excuse to stare at that gorgeous yarn (Opulence DK from String Theory in Tourmaline, if you missed it before and are wondering)!