1. Is the color appropriate to the period and publisher?
Most prints are colored in a typical manner compared to other prints from the same series, by the same publisher or from the same time period. While some prints were colored in an atypical manner at the time of publication, inappropriate color is an initial clue that the color is not original.
2. Is the color skillful and of professional quality?
As most original color was applied by skilled illuminators who made their living coloring prints, the general quality of original color is quite high. Sloppy coloring or coloring inappropriate to the information on the prints are both clues indicating that the color is not original. Many prints colored recently are beautifully and appropriately colored, and some original color is maladroit, but in general a print with poorly done color is likely not to have original color.
3. Does the color bleed through to the verso of the print?
Prints with new color were sometimes inadequately sized, so that the color bleeds through to the back of the paper. This rarely happened to prints with original color.
4. Is there evidence of color being applied after restoration?
If a print has been cleaned, it will often lose any original color it had, especially certain fugitive shades like red. If a print shows evidence of having been cleaned-–whiteness of paper or a chemical smell-–then there is a good chance the color has been added or enhanced after that restoration. If color is applied on top of repaired tears or holes, this is a clear indication that the color is new.
5. Does the print have gum arabic?
Gum arabic is a gummy substance often used in conjunction with hand coloring in order to add depth or texture to the image. This was only used on some antique prints, and even sometimes it will be missing from some examples of prints with original color where other examples from the same series will have it, so the absence of gum arabic does not mean a lot. Also, modern colorists know that collectors often look for gum arabic, so it is sometimes added to prints with new color. All this means that the presence or absence of gum arabic is definitely not conclusive on the issue of original color, but its presence can be a clue that the color is might be original.