In this senerio there are three types of snakes. Coral snakes, brown snakes, and yellow snakes. Coral snakes overpopulate this area and there is quite a number of brown snakes which blend in with the dark sand. There is few yellow snakes since birds spot them easily due to their yellow coloration. All these snakes compete for the same food (rats) but since their are so many snakes the rat population is diminishing and there isn't many birds to eat the snakes
Later on due to heavy rains and sandstorms the dessert sand is now a yellowish color and has a few bodies of water now. The brown snakes are know standing out and getting eaten by hawks in the sky. The coral snake population has not been effected since the climate is still dry and their defense is their deadly venom. But the yellow snakes can know blend into the new dessert environment.
This new coloration of sand helps the yellow snakes blend in with their new environment and they reproduce creating more yellow snakes making the population bigger. The coral still have a stable population with no change since their predators the birds don't want to get bitten. Sadly the brown snake population couldn't adapt and became extinct.
The years past and the desert became drier and the yellow snakes start populating, becoming a little larger, and become immune to coral snake venom. Even though the rats fur is now a tanish color the yellow snakes still have enough food (rats and coral snakes) and less competition for food and can hide from hawks. Due to climate brown snakes are gone, coral snakes are less, and yellow snakes are overpopulated.