You can bleach or dye before or after you dread your hair, it don't matter. Before is better, if possible, as it will be easier to do without your hair all locked up. Coloring before will also help to fry the hair a bit, which will help the dreading process. It's not difficult to dye or bleach once your locks are in, though, just proceed as you would with normal hair and really squish the dye out when washing. It's up to you. Just make sure that your dreads are strong enough to handle the action, which could be up to 3 months or more after you put them in, and don't use the wax in there for a while, as the bleach/dye might not take to it very well. You'd have to wait until the locks are really tight and together.