Air filter turn to black while it is clean?

[cyclon]

Hi.
I installed a new air filter to vauxhall Nova 1990 car
(carburetor).The air filter is a round shape and it is yellow when new.
After just 2000 km the air filter turn to black,but it is clean.
While i didn't drive at any dust situation,there isn't any debris or dust on it,it looks clean,but the yellow filter
turns into black.I though maybe,since the car is very old,maybe soot or something came-out from the engine through the carb,but the inner side of the filter is yellow-clean
and the black is only at the outside part,so it seems that
it is not the case.
Any thoughts what can it be?
Thanks.

[cyclon]

Anyone?