It's not the rain per se, but rather the atmospherics taking place due to the rain clouds. You often get that popping of primary colours during a light rain or just after it. This is due to the way the clouds are affecting the light from the sun. A sort of huge light diffuser.
We see colour via vibration, the same way we hear sound. When atmospherics are just right, due to light diffusion through certain clouds, the primary colours actually vibrate more, causing the eye to see these primary colours in a much more vibrant way. That is why the word vibrant is used, because it is due to vibration.
You will sometimes notice that during a light rain or just after a rain, the greens seem more vibrantly green and the browns of the tree trunks more rich and the blues more blue and the reds more intense. It is because they are vibrating against each other at a different frequency, compared to the way we normally see them in other/average lighting situations.
I love it when this happens and of course it is not just an optical illusion, it really is happening because if you take a photo at this time and correctly expose it, you will see the colours are different to taking that same photo the next day, or even an hour later on that same day.
I love taking a walk in the woods during unusual lighting displays like this. The greens and browns of trees are very vivid as are red flowers for example. Your blue jeans look more richly blue and your skin tones on your arms look more rich. Sometimes this effect lasts just a few minutes and other times it may last half an hour or more. One thing is for sure, it never lasts long enough for me.
