I know optical brighteners are used in most white papers, but are they commonly used in other colors of paper such as greys, pastels, etc?
Some papers have substrates that fluoresce uV and if these substrates, which are cheap, are used then there will be some response to uV even on colored paper.
There are also specialty papers with fluorescent dyes that will actually absorb blue and green light and turn it into yellow, orange or red. These are used for brightly colored fliers or posters because they not only reflect most of these colors but turn shorter wavelengths in the blues and greens into the warmer colors such as yellow, orange and red. These dyes are a type of OBA but not for making a paper whiter but to produce a brighter color.