How to paint Tree Bark

About Marvel Zombies: A Zombicide Game – Guardians of the Galaxy Set, from BoardGameGeek:

The Guardians of the Galaxy Set is an expansion for Marvel Zombies that adds a roster of space-dwelling characters to the game, including the ragtag members of the Guardians of the Galaxy. This band of living and zombie heroes must all contend with the momentous threat of zombie Thanos, as he wields the Infinity Stones to fulfill his mad plan! Thanos acts as a special Enemy spawn that can be added to any mission, whether it be in Zombie Mode or Hero Mode. Every time Thanos activates, he will tap into the power of one of the stones (represented by a special Action deck). This will grant him additional game-altering effects during that Activation. –description from the publisher