Colour-composite images in HSC g, r, i of red spiral galaxies from simulation set A. Simulations consist of the sum of a smooth component, a Sérsic profile with n = 4 and R_e = 2.52″, and a set of spiral arms, accounting for 10% of the total r-band flux. The spiral arm parameters cover a grid of values, as indicated in the panels. The values of the r-band half-light radius and the light-weighted radius of the spiral arm are shown in the bottom right of each panel.