Concerning that idea, maybe they did dress him up brightly colored on purpose, but not to inform others that Chris is 'special', but rather as a ... let's say safety feature.
You know, a kid that's wearing bright red, yellow and blue clothes is easier to pick out in a group, find outside when lost and since Bob apparently paid some girl to look after Chris on his way to the Bus, making him visible to traffic might have been a consideration, too.
And with an autists fondness for routine we get shirts like the "Advice Dog" or the "Horizontal Horror" 15 years later.