From german wikipedia: It was a practice called Buntschießen, ~ multi-color shooting, as German chemical weapons were color-coded. One would start with “Maskenbrechern” ~mask-breakers (blue) , designed to force soldiers to remove their masks, which were then followed by lung-affecting agents (green).

• https://de.wikipedia.org/wiki/Chemische_Waffe#Buntschießen

• https://en.wikipedia.org/wiki/Green_Cross_(chemical_warfare)

• https://en.wikipedia.org/wiki/Blue_Cross_(chemical_warfare)

• https://en.wikipedia.org/wiki/Yellow_Cross_(chemical_warfare)