https://github.com/binji/binjgb/blob/main/src/builtin-palettes.def
Link is the pallette list.
For web export, you can change the default palette in the web files exported under web/js/script.js
Line 30 reads "const DEFAULT_PALETTE_IDX = 83;"
83 is the GB Studio default. 0 or 5 should be greyscale.
Not sure how it works for a usual ROM export to be played on an actual Game Boy. I though it was the same but it isn't.