Preworkout changes colour after a couple of days. Is this normal?

[c234a11]

I just wanted to know if it was normal for a preworkout which is mixed with water, to turn to a black colour after a couple of days,
or if it is a bad tub?
It is a light pink colour to begin with when water is added. However, after a couple of days, what was left in the shaker at the bottom,
turns to a black colour.
I know there is bioperin (pepper) in it, and the flavour is kiwi and strawberry, or is this something else?

Comments appreciated.
Regards,