You'll probably have to set the palette up before you try drawing
in the new colours.

Use the bios interrupt calls to set the r g & b values (in the range
from 0-63 for most cards) for a particular palette colour (in the
range from 0-255 for 256 colour modes).

Then you should be able to draw pixels in those palette values and
the result should be ok.

You might have to do a bit of colourmap compressing if you have
more than 256 unique rgb triplets, for a 256 colour mode.
