Aristotle, a Greek Philosopher & Scientist was the first who initiated the development of colour theory by determining how rainbows are produced in 300BC.
It was his analysis that Rainbow’s are formed by reflection & refraction of sunlight through rain clouds or droplets of water viewed on a particular angle by the viewer.
As the suns rays are reflected through the tiny droplets of water they are refracted at a fixed angle of deflection to the viewer causing this point to be the brightest point in the sky. It appears oval as there are multiple points of the fixed angle that the viewer can see which creates an oval shape due to the angle of refraction. In fact, if the viewer was not on ground level, they would be able to see the entire circle shape of the rainbow. This can sometimes be seen from the sky in an aeroplane or similar. The multi-coloured bans of the rainbow are from each colour having a slightly different angle of deflection & hense a slightly different brightness at each point. The rainbows shape can also change shape as the sun changes position in the sky. When the sun rays are low the viewer can see much more of the rainbow circle & higher sun rays the circular bow is less visible above the ground.
There were few details that were not entirely clear to Aristotle, such as his observation that there can be a second rainbow that mirrors the other but had not figured out the physics of how & why. Also his claim that there were only three colours in the rainbow red, green & violet. Aristotle did not know how the individual colour in the rainbow were formed.