there are no rules.
most watercolorists will employ white papers so that we can use the underlying white for some of the highlights. I mostly use D'Arche 300 pound cream colored paper and I use a "maskoid" (a liquid latex "frisket") to preserve the white highlights as I paint over the area.
Ive often used a tan or grey paper for watercolors and guache mixed media. I would only work in monochrome darks and lights. This technique is called grisaille and is often used to familiarize the student artist with values of light and dark before going full ahead on using many colors.
Grisaille is a good trick if youre a beginner, if not, its still fun to boil down a complex study into just values of dark and light (And usually we use white India ink or guache white over the monochrome shadesof the paper)