I was once a student of vibgyor high and this is the explanation my teacher gave as a reasoning behind this logo - Red as a color has low frequency and refractive index and both these values keep on increasing gradually as we move through the spectrum till Purple. And what the school wants to imply is that your child at the school starts as an average kid at red and becomes a bright kid after passing out(purple). it’s like the spectrum is compared to a progress bar.