Default palette for fourfold displays
fourfold_palette.Rd`fourfold_palette()` returns the six colours used by [geom_fourfold()]. The colours encode the direction and statistical strength of association and are drawn directly by the geom rather than through a fill scale. Entries 1-2 are used when `extended = FALSE`, entries 3-4 for an extended display without adjusted significance, and entries 5-6 for an extended display with adjusted significance. Within each pair, the geom assigns the two colours according to the sign of association and cell diagonal.