Garmin DESCENT MK1 Owner's Manual page 19

Blue: Zone 2.
Green: Zone 3.
Orange: Zone 4.
Red: Zone 5.
Your central nervous system (CNS) oxygen toxicity level.
Green: 0 to 79% CNS oxygen toxicity.
Yellow: 80 to 99% CNS oxygen toxicity.
Red: 100% or greater CNS oxygen toxicity.
Diving 11