All you need are sharpies!
The best thing about sharpies is that you almost always have one and are 10x cheaper than the temporary hair color pods from the beauty store.
Separate a piece of hair that you want colored and with your sharpie run the sharpie along the small section of hair. The more you go over it the darker the color will be. Try doing the top a bit lighter and get darker as you go down to get a more blended ombre effect. You really can't mess this up so have fun!
The best part is that it will wash out after usually just 1 wash.. if you really darken your hair with color it will last up to 2 washes.
*This only shows up on light colored hair or highlights. I've been using sharpie highlights all summer and so far the sharpie hasn't damaged my hair. Remember these are sharpies and probably not intended for hair coloring purposes.