Maybe we shouldn't burden a children's holiday with adult prejudices and concerns. If a child wants to dress up as the Disney princess Moana, is it not because she admires something in the depiction of the character, such as her courage or resourcefulness or strength. Those kinds of traits, which are typically what a child identifies with, are not colour-specific, nor should we encourage children to view them as such.
As for your child becoming a 'force for change' for a child with disabilities, speaking from experience, what most such children want is to be treated as a typical child, to have friends who include them and accept them as they are. They do not enjoy feeling centred out for special concern by other children, however good-hearted.
We need to be careful that our well-intentioned efforts to address social ills are not in fact inflaming them.