Function write_set_palette_color 

pub fn write_set_palette_color<W: Write>(
    w: &mut W,
    index: u8,
    color: &str,
) -> Result<()>

Set one indexed palette entry with ESC ] 4 ; <index> ; <color> BEL.

index is written as a decimal palette index and color is emitted verbatim. Use xparse_rgb to build an rgb:RRRR/GGGG/BBBB payload from 8-bit channels.