Above is a demonstration of a color management technique that comickers have been using to keep pre-press file sizes small for years. Because the human eye (and printers) are more sensitive to high-contrast values, line work is put on a high-dpi layer and colors are generally done at half as many dots per inch on a layer below. I've replicated the technique here by separating the colors from the lines, halving their width and height, and putting them as a background behind the linework image.

Please compare the file size and appearance differences. In some circumstances, this technique might be useful.

This is what the two separate images look like before stacking them: