SUSIE'S REPLY: My, this is a loaded question!
Obviously, if your painting is to be entered into a competition go by the rules outlined in the prospectus. Many shows have strict regulations against using anything but transparent watercolors in the paintings. Rules are rules and its usually not the juror but the board or show directors who do the regulating.
Outside of the competitions.... I think there could be several legitimate choices.
Let's look at a few choices:
- Watercolor is preferably the first choice. It can be painted on using a brush, or by using a stylus in a damp area a signature can be "imprinted or scratched" into the paper. This method allows the signature to blend into the painting. It's easily seen but not distracting.
- Watercolor pencils are dried watercolor in pencil form and are easy to use. If the marks made with the watercolor pencil appears to be sitting on top of the paper,,, try running a damp (not wet) brush over the signature. This will help it integrate with your painting.
- Ink - is used by many artists. It's convenient and easy to use. Preferably, it should be an archival water based ink. (Avoid petroleum based inks.) Some of the archival gel pens manufactured for scrap booking are recommended even in darker areas.
- Pencil is also a reasonable choice, especially if any line drawing is used as an important part of your watercolor painting.
So what do I think about a painting being classified as Mixed Media when you use ink for a signature? For what it's worth, I do not believe using ink just for the signature should turn a watercolor into mixed media.
However, if there is a question, or becomes an issue why push your luck?
More than anything paint it and sign it! Then move on to the next one!