First to say: I'm here to help, and because never heard, seen described bad color problem.
It would be helpful if Rockyone would give more details:
There are 2 BMP files in supplied zip file, and one is with white, other with little less white white
And no word how each is made exactly.
Then in what OS - Win v. for instance , was it ?
"strange ... but not boring for conversion to another format. four bits are correct"
What four bits ? Monochrome is 1 byte = 8 pixels, or 1 bit = 1 pixel. Saved true color BMP is 24 bits for 1 pixel, so waste. You can save it without compression in 32KB file. Instead 760KB . And it will be almost same as Degas PI3, except header.
Finally, screenshots should not depend at all from how Steem is set, in what mode runs, what is Windows screen mode (color depth). It should save emulated Ataris screen buffer - and set colors according to $FF8240 .... - in mono mode that can be direct, or inverted only. Only B/W .
24 bmp is used because Steem can save high-color, Spectrum 512 too. So, on screen can be over 1000 colors at once. Since there is no 16bpp BMP format, it needs to be 24bpp. Although, in case of mono, it is really waste of space, especially as there is no way to have more color nuances on screen in mono mode.
Famous Schrodinger's cat hypothetical experiment says that cat is dead or alive until we open box and see condition of poor animal, which deserved better logic. Cat is always in some certain state - regardless from is observer able or not to see what the state is.