Yeah, I'm with Penlowe. Without being able to see the rest of the outfit to know for sure, it doesn't seem to me like it's a sweater. Even with a ribbed collar, it's not necessarily always a knit - some artists just draw ribbing lines on collars and cuffs because that's what they're used to or they think it looks cool, but it's not meant to be a knit garment or a knit fabric.
If you can find a knit fabric (jersey or otherwise) in that color, it may be as easy as stitching some lines in it to give the appearance. But even if not, a regular cotton fabric can be quilted - you'll interface the inside of the collar, naturally, and then stitch lines on top of it when it's finished. It doesn't even look very thick so I don't know if you'd even want to add a layer of batting inside - I would, if it was a thick rolled collar that looked knitted, but for something that flat, it's not necessary.
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