What makes Skittles change colours in pepsi max and why do the red ones stay red?

Skittles do not change color in Pepsi Max or any other clear liquid. Although it is a common myth, it has been debunked by numerous sources. There is, however, a phenomenon known as "spectrophotometry" that explains why certain objects appear to change color when submerged in liquids of different densities or refractive indices.

When light passes through an object, it is either reflected or absorbed. The color of an object is determined by the wavelengths of light that it reflects. For example, a red object reflects red light and absorbs other wavelengths of light.

When an object is submerged in a liquid, the liquid can change the path of the light waves. This can cause the object to appear to change color. However, this effect is only temporary and will disappear when the object is removed from the liquid.

In the case of Skittles, the red ones do not change color because the pigment used to create the red color is not affected by the refractive index of the liquid. The other colors used in Skittles, however, are affected by the refractive index of the liquid, which causes them to appear to change color.