Yes, you're right, but on a wet filter, you'd be surprised how much difference that slight discolouration makes. I don't know if they have changed, but k&n's used to be bad for this, worse than other makes. The best type was the old vortex oil filters on early landrovers, a big cannister full of coarse wire wool, with an inch of engine oil in the bottom. The oil wicked up onto all the wool. The air was directed into a vortex through the wool and any dust particles stuck to the oil, any big bit fell to the bottom. Good enough for desert conditions, no loss of airflow AT ALL, and only needed engine oil topping up every few years. Only downside is they were about 18" high by 10" round and weighed a ton!