Combine the cocoa powder with the water thoroughly, stirring until the powder is completely dissolved.
Add the corn syrup and food coloring. The amount depends on the consistency and color you want from the fake blood. Adding more corn syrup will make it stickier and take longer to dry. Add the green or yellow food coloring with the red, if you want the fake blood to have a bruised look to it once it dries on skin. Combine all the ingredients and let the mixture sit for about five minutes after stirring.
Skim any bubbles that have risen to the surface with a spoon and discard. Pour the mixture into a separate container. You can apply the blood right away, or let it sit for a long period of time. The longer it sits, the more cocoa settles on the bottom of the container, creating the effect of blood coagulating.