Black isn't always just black ink. 100% black ink with nothing added is "plain black" but when you add varying amounts of cyan, magenta, or yellow you get shades of rich black such as cool black and Designer black. There's even a black known as Photoshop black (why? because it's the default black in Photoshop).
Stumbled across this recently. MyHexColor wants to sell you a color for $1. Not just any color. Your color. Created by Damien van Holten and Bart Veraart, the site is a project aimed at "exploring what colors mean to people."
While working on some paper definitions I ran across a series of useful PDF files provided at The Paper Mill Store. One of those PDF files is The Envelopery.
It's an 11 page document devoted to just about anything you need to know about envelopes. Focused on US envelopes, it covers postal requirements, standard sizes, custom envelopes, and lots of charts and illustrations of various envelopes. Nice reference to have.