Pattern specifies Phildar Perle 5, which seems to be a crochet cotton & asks for 10:10:11:11:12 x 40g balls, plus a ball of each contrast colour for the stripes in the V-neck ribbing. I don’t know what the exact yarn equivalent in knitting terms, so hazarding a guess at fingering (4-ply) - I’m currently swatching with a fingering and seem to be making gauge.