Download
this patch, then open "ow_mario.asm" and change "!ow_mario_palette" to $0, and also change "org $0487CB|!bank" to "org $0487CB" and "org $0489DE|!bank" to "org $0489DE" (the file is not supposed to be applied directly so it doesn't have the !bank define, but it doesn't need to be used there so you can just remove it from those). The patch seems to have a small issue with Mario's walking animation where it replaces some frames with the blue bird sprite, it seems like you can fix that by replacing
Code!walkb2ul = $4C
!walkb2dl = $5C ; this frame is walking away from the screen
!walkb2ur = $4D
!walkb2dr = $5D
with
Code!walkb2ul = $20
!walkb2dl = $30 ; this frame is walking away from the screen
!walkb2ur = $21
!walkb2dr = $31
Then apply it with Asar (don't apply the other asm files!). This will make Mario use palette 8.
To also change Luigi's palette, apply this hex edit with Asar: