This mathematically inclined hat has two options - a folded hem or ribbing to start, then simple colorwork the rest of the way up.

As shown:

BC Garn Semilla Melange 100% GOTS certified organic wool 191 yards (175 meters)/50 gm, one skein each: Chocolate-13 (Main Color) Rust-12 (Color A) Aqua -08 (Color B)