Orange Spots In Underwear. Why Would These Appear?

Under the use of a black light and orange tinted eye wear , I ve found many orangey, spots, smears, and dots in my husband s underwear. There is no smell of sex in any way but I m curious as to why he would have these when urine stains tend to be more yellow or a whitish color. He doesn t take muti- vitamins or any type of medications whatsoever, and he swears that he s always been faithful, however these same colored stains also appear on his underwear even after he and I have been intimate.