My days of knifing people are over. (What can I say? I'm on my third strike.) But when I get nostalgic, there is a way to make fake knife slashes on a person that look real. And better yet, you can use it so it appears right as you draw a (fake) knife across a person's skin, too.
First, get some potassium thiocyanate, or KNCS, and mix it with water — a few grams in a few milliliters will do. Next grab some iron nitrate, or [Fe(NO3)3], and put that in a separate container of water. Keep the two containers and stirring implements strictly separate. Paint your arm or hand with the potassium thiocyanate solution. Dip a knife in the iron nitrate, and then draw the knife lightly across your arm.
Neato, huh? Here is what's going on: When the potassium thiocyanate hits the water, it separates into a positively charged potassium ion (K+) and NCS-. The iron nitrate separates into a triply positive Fe+3 and three singly negative sets of NO3-. The potassium and the NO3- we will never hear from again in this article. However, the negatively charged NCS- and the positively charged Fe+3, get together and form FeNCS+2. This is what makes the solution blood red. Many people recognize iron (Fe) as what makes our hemoglobin bright red. Essentially, we're bringing out the red in iron, but finding a way to do it without the hemoglobin.
There is a complicating factor here. The Fe+3 ion on its own is a light yellow color, and the two components of our fake blood separate as well as combine. What we're seeing is a group of chemicals reaching an equilibrium, and that equilibrium can change. Add some acid, for example hydrochloric acid, and the iron gets snatched away from the FeNCS+2, eliminating the red color and turning the solution clear or very light yellow.
If you're a chemistry nut or a horror nerd, you can test different pH levels to see what will turn the solution red, yellow, and red again. But perhaps, if you're using hydrochloric acid, don't test it out on your skin.