Picture from Randy Mosher's Book 'Tasting Beer'. This is an excellent book to get if you are interested more in learning more about your beer, how it is made, beer culture, tasting and more. You can purchase a copy here: shorturl.at/hqrE5.
SRM - Standard Reference Method - this is the Americal colour standard.
ECB - European Brewery Convention - equates to roughly double the SRM.