I have a cashmere sweater, never worn, in a peach color. I am wondering what might happen if I dyed it. It was inexspensive but still-would it dye evenly or would i wish i had left it alone?

I'm not much for dyeing but I have had friends over-dye sweaters with great results.

Since cashmere is a protein fiber, you would probably even be able to over-dye it with food dyes such as Kool-aid, food coloring, or Wilton's cake dyes.

I would make sure that there aren't any stains on the garment since that could affect how the color is taken-up. Because this already has an existing color, you might see if there are some stray strands that you could test-dye to see what the final color could be. Or you could throw caution to the wind, pick a color you like and see what happens when you over-dye your sweater. Worst come to worst, you could keep over-dying it until you get close to black. <G>

As KT noted, cashmere is a protein fibre so make sure you use dyes that work for protein. Those listed will surely work, but be careful, they will also dye your counter and anything else they contact.
It would be extremely rare if the sweater was 100% cashmere, so check to see what else it's blended with, cotton? acrylic? You may then want to use one of those all purpose dyes that you can buy at the fabric store.
Best of luck,