Found a resource might be helpful at last.
https://stackoverflow.com/questions/...-grayscale-bmp
From the first answer, though the question and the code is to make a gray scale 8bit bitmap from colored
Code:
var entry = bmp.Palette.Entries[index];
var gray = (int)(0.30 * entry.R + 0.59 * entry.G + 0.11 * entry.B);
newPalette.Entries[index] = Color.FromArgb(gray, gray, gray);
This line is adjusting the palette (i was looking for this word!!) i used lookup table or predefined color table.