This was taken from http://www.funnypracticaljokes.com/

Food coloring is a great way to get someone to throw out his or her milk well before its expiration date. Just add a few drops of black food dye to a jug of milk and your friend will think it has curdled. I used to like to taint my milk and then drink from the jug with others looking on in disgust.

Here is another great trick if your friends have children that drink Kool-Aid or juice. Empty the Kool-Aid/juice into the sink. Fill the container back up with water and add a few drops of the appropriate coloring dye to make the water look like the previous contents of the pitcher. The kids will complain to their parents that something is wrong with the Kool-Aid. When I did it, my friend told his kids, "you are just imagining things, I am not going to waste good money and mix another batch just because you kids want more sugar in your Kool-Aid." He finally tasted the phony mixture and realized something was amiss, but not before making his children feel inadequate.