Raster Images:
 Image Processing
  Doing Color Expansion or Reduction

Automated color reduction:

PaintDither property

General color expansion or reduction:

ColorRes method

ColorResList method

ConvertSignedToUnsigned method

Support for dithering to a custom palette:

UserPalette property

Specialized color expansion or reduction:

ConvertToColoredGray method

Grayscale method

GrayscaleExt method

Halftone method

IsGrayscale property