If you have problems reducing the colors on a raster image try the following.

Combine automatic and manual reduction

Use a combination of automatic and manual color reduction.

For example, if you have a 256 color image that you want to reduce to 16 colors, you could try reducing it to 32 colors automatically and then making the final reduction manually using Palette Menu > View/Edit Palette.

Rescan the drawing

Color reduction will not work well on muddy, wishy-washy colors.

Experiment with your scanner’s brightness and contrast settings and rescan the drawing so that the colors on the raster image are as high contrast, vibrant and saturated as possible.

Thin the raster image before reducing the colors

Some raster images have lines with “haloes” that are not the same color as the main line. To resolve this problem, try thinning the image before you reduce the colors.

Despeckle the raster image after reducing the colors

Despeckle the raster image after reducing the colors to make the reduced color areas more uniform.

Vectorize the image using the Vector Colors to Raster Colors method

If you can’t reduce the colors acceptably or it is taking too long, try vectorizing the raster image using the Vector Colors to Raster Colors method.