The best time to paint grass is after it's freshly mowed. This way, you don't mow off the color.
Choose early morning, perhaps there is a light dew or a light frost present. This light moisture may help for even coverage of the Endurant turf colorant. The best time to paint grass is when the wind is not intense, to avoid accidental overspray. Also choose a time when there is not a heavy rain in the forecast for up to 24 hours.
As for whether to paint grass before grass is fully dormant or two wait, that is a personal preference. Many choose to paint grass while it still has some color. They will apply Endurant turf colorant at a lower rate when there is already some green on the grass. Then they may apply another light coat when grass is fully dormant. This coat will in almost all cases last throughout the dormancy phase. The grass may be painted again, or a pigment or Flex may used, when transitioning in and/or out of dormancy.