Your filter is only going to filter the air when your air system is running. Your system is probably working a little more, and in return capturing more particles thus looking "dirty".
Or
Are these the filters you have always used? If not they may be capturing more then your old filters were capable of.
If it looks dirty that is a good thing! The filter is working as intended.
Or
Are these the filters you have always used? If not they may be capturing more then your old filters were capable of.
If it looks dirty that is a good thing! The filter is working as intended.