Really sepia tones on a map means it's aged, which also means fading, yellowing and loss of color. So many maps that are supposed to depict new locations seem to require hundred year old maps because of an over reliance on sepia tone colors. Regarding colors on illuminated documents, check out the Book of Kells. If you want traditional illuminated maps, you definitely want colors. If you want your map to look old, then, and only then consider using sepia tones.