Why is it important for fabric dyes to wash fabrics after dyeing them?

Question

Why is it important for fabric dyes to wash fabrics after dyeing them?

Answer 1

The colors will continue to bleed a little after it's been rinsed.  It needs a good washing by itself to keep the colors from ruining your next load of laundry.

Answer 2

If you don't wash fabric after you dye it, loose particles of dye which remain will rub off of the fabric onto anything that touches it. For example, if you try to dye an upholstered couch with Rit dye, the dye will rub off onto the clothes of anyone who sits in it, even if the dye has been allowed to dry completely.