For Backgrounds you can only use this 4 colors.
#071821
#306850
#86c06c
#e0f8cf
The most shiny green is for transparency on sprites. In that case you can only use this other 4 colors.
#071821
#86c06c
#e0f8cf
#65ff00
https://www.gbstudio.dev/docs/backgrounds/#tile-requirements