Basically, the game rolls twice.
1. Rolls for rarity
2. Rolls for what girl you get within the rarity that was rolled. This is what the spotlights affect.
In other words, just a fancy way of saying "if you are lucky enough to get a 5-6 star girl there will be a higher chance for her to be these girls!
Your chances to get a gold/rainbow doesn't increase the slightest.
That said, there is a button that shows the drop rates for the rarities, those being 0.5% for rainbows and 6% for golds.