Natural dyes

DS has a really nice pair of off-white linen trousers that he really likes, but doesn't wear because he's worried about getting them soiled. So, wondering if anyone has any tried and true methods for using a natural dye such as black tea, and be able to fix the colour so it doesn't run. I used to know this, but I can't pull it out of the memory banks!

Thanks.

After poking around for a while, I found the right combination of words to get this hit on google:

tiinateaspoon.blogspot.com/2008/04/dyeing-with-tea.html

In case anyone else was interested.