The illustration shows how the making of a rainbow begins when sunlight strikes the front of a raindrop. The entering white light is refracted or bent and separated into it's component colors. These colors then bounces off the back of the drop and are bent again as they leave the drop behaving as if they had passed through a prism.
Why are the different colors formed? Each of the colors present in white light are bent a different amount in passing through the drop. The red bends the least and violet bends the most. The rest of the colors lie in between. The result a red, orange ,yellow, green, blue, indigo iolet color spectrum. The only portion of the colored light you get to see is the color directed at an angle that actually strikes your eye. So, each drop brings one and only one color to your eye. Two people located at different positions will see different colors coming from the same drop. A drop that brings you a speck of red light may in fact bring me a speck of green or violet light. Each drop of liquid is also falling and as it's position changes the color that reaches our eye also changes. So a single drop in it's fall from the clouds to the earth can and does bring all the colors of the spectrum to our eye. The colors are brought at different times. The rain drops just continues reflecting and refracting as they fall to the earth. The location of each color reflected off the back of the drop and striking your eye is always at a fixed angle measured from a line between your eye and the sun. This angle is approximately 42° measured to the top of the red band and approximately 40° to the bottom of the violet band.