The Technology & Maintenance Council (TMC) of the American Trucking Association (ATA) has set helpful guidelines for engine coolant color determinant upon the coolant chemistry formulation. TMC RP 365 breaks it down quite simply into the following delineations:
RP 302C: Conventional Low Silicate is recommended to be green
RP 329A: Fully Formulated Ethylene Glycol is recommended to be purple/pink
RP 330: Fully Formulated Propylene Glycol is recommended to be blue
RP 364: Organic Acid Technology (OAT) is recommended to be red
Learn more about engine coolants here.