In reply to Tigger:
I wouldn't use bleach, it might degrade the fibres. Just wash, reproof and live with the staining.
Or camouflage it by doing what my school friend did when we lent him our family Vango Force 10 - leave it in the boot of your car, painted with bitumen, alongside a leaking petrol can and when brown liquid soaks into the tent and leaves it looking like a psychologist's inkblot test, just hand it back without a word of regret or offer to replace....