I've been in settings like a religious school with a very strict dress code, as in hemlines well below the knee, and no necklines cut any distance down the back, let alone the front. But I have never actually come across anyone condemning brightly colored clothes. I have met a few rather sad people who always dressed in very drab shades. In some cases, it appeared to be a symptom of depression. In a few cases, my guess was that the person did feel that bright clothing would be too modest, or "non-spiritual", for them. Then again, I suspect that a couple of those who told themselves that they were being "spiritual" were actually displaying symptoms of depression.