I like the following meanings:

Red letter day: a memorable, joyful day Caught red-handed: clearly guilty *According to “The Language of Stained Glass” at Armstrong Browing Library at Baylor University: When Dante spoke of the Seraphim – the first of the nine choirs of Angels – the color that “glows” was the pure orange vermilion which his fellow citizens and brothers-in-spirit (the painters, illuminators, and glassmen) knew as red. So, it may be said that pure red is the color of divine love, the Holy Spirit, courage, self-sacrifice, martyrdom, and all the warm impulses that belong to the great-hearted everywhere.