Answer:
The doghouse was painted green because they couldn't find any red paint.
Step-by-step explanation:
According to the passage, the narrator and others were not able to find red paint for the doghouse. That is the reason why they used the green paint they already had. Though they did have money to buy red paint, using the green one seemed to make everyone happy, including the poodle, Pierre. So, simply put, the doghouse was painted green because there was no red paint to be used.