I‘ll just copy the explanation from one Hamish Gill on 35mmc

The reason it goes red is because the when you shoot colour film the wrong way round, the red layer is facing out first instead of last. As a result, the light hits the film red side first, and since this isn’t how it is supposed to be used, you get red photos. In theory, the more you expose it, the more light will travel through to the layers beyond and you will eventually start to see more other colours. In reality though, results tend to consist of pretty much red, orange and yellow. Or, in other words, sort of post-apocalyptic looking