Adjusting the Consistency
In many cases, fake blood is simply too thin and looses any realism that it might have had because it flows more like water than blood. This can be corrected by adding clear corn syrup to the fake blood a little bit at a time. If you accidentally make the blood too thick, it can be diluted with water until you reach the desired consistency. Because corn syrup is a natural food ingredient, it will not add toxicity to any fake blood that is to be put into the mouth.
Adjusting the Color
When problems occur with the color of fake blood it is generally because it is too red. Adding food coloring to the fake blood is an easy way to adjust the color, but it's important that you understand exactly which colors to add. Begin by adding a few drops of blue food coloring to adjust the color of fake blood that is too red, mixing it well in order to make sure that the color is consistent. If the blood appears a bit too purple after adding the blue, begin adding red food coloring by the drop (making sure to mix well between each addition) until you reach the desired color.
Adjusting the Richness
Another common problem with fake blood is that it is often too bland looking, even when the color and the flow are correct. Real blood has a certain richness to it that can't be emulated by clear liquids and food coloring; the fake blood will still appear watered down even if it flows correctly. This desired richness can be added easily, though, by mixing in cocoa powder a pinch at a time. Make sure that you mix it very well between additions, checking the fake blood to see if it looks the way that you want before adding more. The cocoa powder not only helps emulate the richness of blood cells, but if the fake blood is designed to go into the mouth then it will also add a slight chocolate flavor to make it more pleasant.
