Creatures evolve randomly in some aspects and in other aspects they don’t. This is a random evolution. It may have started with the color change and then they were easily seen and easily eaten until one evolved to be poisonous or it may have started with the poison and then the color change which is more likely in my opinion. If caterpillar bob evolved to be bright red randomly then all the caterpillar bobs would just be a buffet for the birds until another random evolution occurred and caterpillar bob now has poison in his system. Eventually the birds associate caterpillar bob with pain or sickness and stop eating him.

More likely caterpillar bob already had poison and then evolved the bright red color. I’d be already had the poison then being easily spotted by the bird isn’t a big problem because the bird won’t consider him food.

Milk snakes evolved to look like coral snakes. They aren’t venomous like the coral snake but they benefited by looking similar to an animal no other animal wants to play with. It was a random evolution with a big up side for the snake.

An example of non-random is the giraffe. Newer evolutionary theories say that the giraffe stretching its neck up to reach the higher leaves in the taller trees contributed to its neck growing longer.