strange colors during boot

[fxkl47BF@protonmail.com]

i set up bullseye on an old machine that has builtin vga display
i use a vga to hdmi converter
i don't know if that is relevant
during boot everything works ok
i see the grub screen and then initial ramdisk
they are black background and white foreground
it is, i think, when it finishes the initial ramdisk that the color changes
the foreground color stays white but the background changes
it changes to different colors each time i boot, green, purple, pink, ...
some combination are impossible to read
where should i look for the culprit