The film base used for 135 film usually contains a dye which is part of the plastic and not soluble in water or any processing solutions. Other formats usually do not have a similar film base. In any measurement of base fog the presence of this dye must be taken into account since the density it contributes is not fog. For many years the film base used for Kodak Plus-X 135 had a very dark blue color. No amount of washing could remove this dye.