random filled tile from a grid pygame

I hope someone could help me!

I want to make a reflex test game. I've made the grid it's interactive, when I click on a square it turns green.

But the idea of the game I would like to make is that the moment the game starts there will be a random square filled with green, if the player takes too long to click on it or he misses it, the square he clicked has to flash in red. The game ends when they reach certain score or if they run out of time.

But I'm finding it difficult to make the squares random and to fill them with colour! I really hope that there would be someone to help me out!