There's probably pink goo in there and when stretched out, the green rubbery membrane gets thinned out as well, making it easier to see through, thus, it looks like it changes to pink.
@Mydillydink, these were like my internet. And when your lil sis pops one you get in trouble for biting her on the arm at the store even though it wasnt even hard
@Mydillydink, Not sure, i just remember my babysitters kids having them. I just found out they are called infectious disease ball or just squishy mesh ball and can be purchased for 1.99 on amazon.com. i hope I've been helpful :)
Why... Why does the background move? That isn't just me seeing this, right?.. RIGHT?!