Example in spanking art
Here is an example of the colorization of a vintage black-and-white spanking photograph, and a modern digital spanking drawing:
Traditional colorization is done using watercolor, crayons, colored markers, colored ink, or other paint. When colorizing a drawing on paper, non-opaque colors are preferred as they leave the existing black lines and areas largely unaffected.
In cel-animation, the lineart is first drawn onto the front of the cel (transparent foil). Then the colorization is done with opaque colors, but from the back of the cel. This makes painting the areas within the contour lines easier, and leaves the inked lineart completely unaffected and pitch-black. See also cel shading.
With modern software, digital colorization is much easier and best done by working in several layers. The lineart remains uncolored in one layer, and the color is added in other layers. There can be one layer for all colors, or each color (or part of the image, such as foreground and background) can have its own layer. The color layer(s) can be arranged "above" or "below" the lineart layer and different mixing modes can be used. In Adobe Photoshop, the "Multiply" mode is often a good choice for colorizing black lineart.