how do u clean old dirt/grime from dishwasher racks?

I didn't realize it was dirt, until someone pointed it out to me. We rarely wash dishes using the dish waster, but we do store dishes in them to dry off. Now some of the wrack(which is white), look sort of brownish. I checked for chipping of paint, but didn't find any. Rust is red right? It would come off on dishes or my hand too right? This is y I'm thinking it's dirt. I have no idea how to clean it though. Soap and water with scrubbing wont do...Ive tried in vain. The person who pointed out my dirty dishwasher gave me no pointers either. i'd like it clean...especially since actual dish washing is a rarity for the machine. Please do help me with this. Also, if there are any natural remedies, that would help, thanks.