Have you ever put a Tiffen on a white sheet of paper? That yellowish smear.
overed by a filter.
I'm no expert, but I read somewhere that fluorescence was used to whiten some materials like paper and some fabrics which are somewhat yellowish. Some aditives can absorb UV light and reflect it as blue light, which mixed with the natural yellow tone gives a more luminous white. So, if you put a UV filter on a sheet of paper shouldn't it always give a more yellowish tone?
By the way, I don't use UV filters except in extreme conditions.