Minecraft wiki entry for Item_modifier, associated text: set_banner_pattern—Adds or replaces banner patterns of a banner. Function fails if invoked on a non-banner.
 patterns: List of patterns to add or replace with.
: A pattern.
 pattern: The pattern type. For example, square_bottom_left.
 color: The color for this pattern. One of white, orange, magenta, light_blue, yellow, lime, pink, gray, light_gray, cyan, purple, blue, brown, green, red, black.
 append: Optional. If true, the patterns will be applied on top of the banner's existing patterns.