This pattern is knitted in the round using slipped stitches to create the ovals, which means you only knit with one colour in each round.
If you intend to make your own cushion pad to go inside, it doesn’t matter what gauge yarn you use, just measure the final size and make the pad slightly larger. Instructions are given for making a square size, my cushion is oblong because I went on knitting until my coloured yarn ran out. I have reused handspun wool yarn from a Jacob sheep, so my colours are the natural deep brown and the variegated shades, with the white wool dyed with daffodils to make it yellow, then half the yellow was modified with iron to make it green.

Here are the amounts of each colour yarn I used to make the oblong cushion, the weight is about worsted/heavy dk:
Brown (B) 120g approx 240m
Variegated (V) 90g approx 180m
Yellow (Y) 50g approx 100m
Green (G) 50g approx 100m