At its core, The Handmaid's Tale's costume design is a matter of status. Each handmaid is given a red 'uniform' - a scarlet letter that distinguishes them as 'other' while making them easily identifiable should they attempt a futile escape.
In this world, women are second-class citizens; many of those capable of bearing children are forced into sexual slavery as 'handmaids' impregnated by the country's leaders in a dehumanizing ritual.