The default palette should be high contrast for white and black backgrounds, to pass accessibility color contrast requirements. Ideally there are at least 3 palettes, the default being general purpose, one for black ground and one for white ground, and those can have flags for dark mode and light mode.
Technical
The font must support the COLR table.
It can use COLR v0 or COLR v1.
If the font is static and uses COLR v1 it must have an SVG table equivalent to the COLR table
Add SVG to a font with COLR, or vice versa, using maximum_color from nanoemoji
COLR v0 is widely supported; SVG is not required for COLR v0 fonts
If the font is variable it must not have an SVG table (because SVG does not support OpenType Variations).