MVHR filters should be checked every three to six months for signs that they need replacing. There are two in each unit, one filtering the air being extracted from the house which is normally a G4 filter, and the other filtering all of the air coming into the home, which is an F7 filter (a higher grade for better protection). The unit should be switched off when the filters are checked to make sure that no dust gets into the fans.
On the intake and exhaust grilles it's likely that small insects, carbon dust, pollen and airborne particulates will have turned the filter dark grey or black. For the filter that's protecting the home, the filter may be grey from dust that's been picked up from inside the home.
To the right is a PAUL Novus filter after three months in a Central London home.