Why is my shower gel changing its color?

Hey,

If someone is able to enlighten me on this one, it’d be great!
A while ago I bought a marshmallow shower gel, in a plastic bottle, with a pink package. When I first saw the shower gel, I was a bit disgusted because it had a yellow-ish & pink color. Anyways, I thought it was because of its marshmallow scent.
After I ran out of it, I removed its pink plastic package, and kept the bottle because it had a dispenser and it was really easy to use. I thoroughly cleaned the bottle, then I poured some other shower gel in it. This second shower gel was completely white. Then, it slowly started to get the same strange yellow-ish & pink color as the original one.
Does anyone have any idea why is this happening? Should I be concerned (maybe the plastic package is unsafe?)?
Thanks!