How do I get my carpet to look clean?

I know my carpet is clean, I vacuum atleast once daily, most of the time 2 or 3 times (I'm a little OCD) and I shampoo the carpet once a month. It's kind of a light blue color and for some reason it always appears dingy, never clean. I've tried to spray the stain remover before and let it sit 5 or 10 minutes before I shampoo, I've tried different carpet soaps, I always vacuum before and after I shampoo, I just don't understand how it never looks clean!! I'm not willing to replace the carpet because I rent the place, but the landlord doesn't see a reason to change the carpet, it's really not all that old, we've only lived here 2 years. Does anyone know any special (cheap) fix for a not so bright carpet? If I put a cap full of bleach in the shampoo tank, would it harm the carpet or my shampooer? Oh yeah, this is a steam cleaner shampooer if that helps any.