Hi Adi, great question! That's strange, isn't it? This is a tough one to explain, but I'll try to keep it simple.

First off, what is a rainbow? Rainbows usually occur on a day when there is sunshine and some water--like rain, mist, or even a sprinkler. Why is that? Well, as sunlight passes through the tiny water droplets in the air, it bounces around inside of the droplet and then separates into what what we commonly refer to as Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV). The picture below shows this nicely!

Rainbow

What's so special about each of those colors? Well, you have to use your imagination for this one, but trust me: every color that you see (for example, red) is actually a wave traveling through the air! Each color looks different because it is a wave with a specific energy. For example red light is a low energy wave, while blue light is a high energy wave, but both travel through air at the same speed (the speed of light!).

What's the energy of sunlight then? It looks white, right Well sunlight is actually all of the visible light combined together. Well it looks that way, but it's actually all the colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet) added together!

So then when sunlight passes through a water droplet, because each color has a different energy, each color passes through the water droplet at a different angle. This picture in the link above shows this idea really nicely.

Now back to your question: why are rainbows always in the order of ROYGBIV? The answer is that red light is lowest energy, so it is reflected at an angle that shows it at the top of the rainbow, then orange is the next lowest energy so it's next on the list, and then yellow, and so on, and so on until we reach violet--the highest energy color! I hope this helps :)