With whites you can soak them in stong tea. I would assume with colors you could mix some bleach with water and add to the washer (after it's full of water) and then put the curtains into that. Good Luck.
Susan in BAma

Depending on the fabric, you might also try seeing how they'd look if you flipped them over to the "wrong" side.

the fabric is the same on both sides. i was wondering if bleach would work. i'd like to lighten them up a bit, whereas the tea would darken them, i think. thanks, erin

RIT makes a dye for "aging" fabrics that is permanent, while tea will fade out.

They might have a "white-washer" for them, but careful bleaching can do it for uyou.

If you use bleach, be sure that the curtains are very wet before you put them into the agitating tub of bleach water. A bath in Chamomile tea will "yellow" them, which will make them look older.

if they're to dark, 'white dye' or a weak solution of white craft paint (thinned to 'skim milk' would be way better for your purposes than the bleach- if it's strong enough to affect the color, it's going to weaken the fibres...and worse, there's NO telling what the colors will turn out- often, it will pull only the reds, or only the blues, out, and leave you with something scary.