Zombies

In 1913, in Carlton, Pennsylvania (shot in Bulgaria), the cruel owner of the Carlton mine exploits poor immigrant children. In order to excavate a new shaft quickly, he employs a dynamite charge, but the explosion causes the mine to collapse, burying a large group of the children alive. Following his later trial for wilfully causing the death of his workers, Carlton is acquitted and the mine closed down.
In present day, eighty years later, Karen Tunny has just lost her husband after a long period of terminal disease and has inherited his birthhome near the since-abandoned Carlton mine. She moves to the house with her daughters, Sarah and Emma. The three stop by the local market for supplies and are told by Walter, the shopkeeper, that he doesn't deliver to the area they live in. While driving, Karen has a near miss with a man crossing the road. She exits the car, looking for the man, but he's nowhere to be found. They arrive at the house and Sarah points out the blood on the door while Karen declares it's just "paint."
When Emma hears children giggling, she leaves the house, following the sound of it. Karen goes out looking for Emma and finds her in an old mine. As they try to find their way back to the house, it becomes nightfall and they get lost. They find a house, which is occupied by Hanks, and enter. Karen is advised by Hanks to stay at home during the night, and he also tells her that there is no need to thank him for the blood smeared on his and the Tunny's door.
William Carlton, the last surviving heir of the Carlton estate, which has owned the mine since 1913, is hungrily devouring property and kicking people off his own property. Meanwhile, the zombie children begin to kill, which is dismissed among the community as disappearances, though it is hinted that most of the community is aware of the presence of the children.
As it turns out, the Tunny and Hanks families are relatives of the zombie children who happen to leave blood relatives alive while killing all others. Emma, who has had friendly contact with a less-violent zombie named Mary, informs her mother that the zombies won't eat her, and that Mary would not directly hurt her mother (who is not a direct Tunny blood relative), but passes on the warning that the other children might. Karen finds some old family photo albums in the basement that contain pictures of her late husband, as well as the Tunny and Hanks children who died in the mine disaster, thus revealing that the family is related to some of the children who had died.
Karen and Sarah leave the house to go look for Emma. As they exit the mine, not being able to find Emma, they become surrounded by a dozen of the children. They escape, with the children pursuing them, and find a car passing through with Carlton inside. They enter, telling him to drive, but the tires are slashed before they can pull away.
Karen and Sarah run to Hanks' house, unsure of what to do. Soon enough, Karen figures out that Hanks' blood has a supposed repellent effect on the zombie children. As both Hanks and Carlton attempt to shoot the children, they realize the bullets are ineffective, and run to the barn.
Hanks realizes that, as he and Emma are direct blood relatives, it turns out Mary has an older brother that's also a Tunny, and Karen is in some way protected by Emma's relationship with Mary. The children are really after Carlton, as they blame his family for the mining accident that killed them.
After Carlton is killed by the children, Emma remarks that they won't be hurting anyone anymore.
At the end, the Tunny family drives away and reveals that the house has not been sold. Now, Mary lives in the house, with the bear that Emma gave her, with a few of the other children as well

Question: What does Karen think the blood on the door really is?
Answer: Paint