When Fred and George Weasley help pilot the car that rescues Harry from the Dursley residence, they are wearing wonderfully striped knitted vests. Unfortunately for knitters, the pair are still very much background characters, and so there aren't many clear screen caps of the vests, nor is there a pattern.
What is clear, is that the vests are striped, and a combination of tans, olives, and browns. the ribbing at the sleeves, neck, and waist is olive, and the stripes within the vest itself appear to be knit with a seed stitch texture.
As I said, there's no pattern written for this vest as of yet, but any men's vest pattern could be used, and the strip pattern worked over it to give the look.