For the most basic fake blood, a beginning base is required: Mix 1 cup of corn syrup in a bowl with 1 tbsp. of water, 2 tbsp. of red food coloring and 1 tbsp. of yellow food coloring. This is where the trial-and-error process begins. If the color is not right, try adding blue coloring. If it is too thin, add ice cream. Also, if it needs to be a less runny consistency, try adding hair gel.
For more realistic-looking blood, especially for a film project, try mixing 1/3 cup of water with 5 tbsp. of corn starch, adding 2/3 cup of corn syrup and approximately 4 tsp. of red food coloring and 3 drops (about 1/8 tsp.) of green food coloring. The corn starch will give the blood a different consistency. Real blood is more on the brown side, so adjust the color as needed.
If the blood needs to stay on skin and not run, you will need to add hair gel. Sometimes a little peanut butter can work as well. If you want the blood to be thicker and less transparent, try using white glue instead of the corn starch. Again, you may have to make several batches before you get it right.
When working with fake blood for the stage, remember that liquid food coloring usually stains. So be certain to have a meeting with your wardrobe supervisor prior to testing any of the blood on clothing. You may want to use food coloring powder instead because it comes out of clothing easier. This may also require buying several of the same items of wardrobe if it is an ongoing production. Also if blood is needed for a very brief scene, you may be able to get by with dish-washing liquid and red food dye. It is a quick, bubbly, blood alternative. Finally, if the blood must be edible, as in a vampire story, add red jam, chocolate or mint, and under no circumstances should you use white glue or hair gel.