I've come across a quote by Teddy Roosevelt which shows his admiration for Alexander Hamilton. It uses the phrase "the touch of the purple," but supplies little hint as to what it means.
Roosevelt saw in Hamilton “the touch of the heroic, the touch of the purple, the touch of the gallant.”
The best explanation I've found is a possible religious meaning, gleaned from the phrase's inclusion in this Wikipedia article. I am also aware that purple has a connotation of royalty or nobility in that, some centuries ago, purple dyes were extravagant and therefore used as a symbol of wealth and power. I'm not entirely convinced that either of these are the sense that Roosevelt was invoking.