WebArt Designer supports two methods of color reduction. These are with and without dithering.
When dithering is selected, the quality of the image is high since the processing resolution is high. When you do not use dithering, the closest color is used. This is good for line drawings and images with relatively few colors.
A photograph is reduced to 216 colors:
Dither method "Yes" "No"
A line drawing is reduced to 27 colors:
Dither method "Yes" "No"