To jump onto "Newton's party"
if the bag is white, you'll see it's dirty, so you'll clean it off, so it will actually be cleaner
at least that's how it works with my white sleeping bag
maybe re-sale value is less important than usability
but, like I said, maybe the real factor is white doesn't absorb sunlight so it doesn't dry out very good in the sun
Actually, my white bag has a black underside so I just face that to the sun and it probably dries okay