A rainbow displays the colors red, orange, yellow, green, blue, indigo and violet in respective order. Other things as well just like mist, dew and spray can cause its formation. To explain it further, when light strikes the surface of a water drop, it changes speed thus causing it to bend.
It is refracted as it enters the water and then refracts again as it leaves the droplet. The outcome is light being reflected in varying angles, creating a rainbow.
Light travels in varying waves where the length of each would depend upon the color. When light retracts, the different colors refract and bend in different amounts as well. This is the reason why we see different colors in a spectrum when there is a rainbow.
To set up this experiment, take a bowl of water and place a mirror in the water. You want the mirror covered half way and at a angle. I use tape to hold the mirror in place. Then turn out the lights and turn on your cell phone light and angle towards the mirror. You might have to adjust the angle of the mirror to get the perfect rainbow.