The pink liquid that spills over Pink Guy, covering up the black? I don’t think that’s him before he got Pink, i think that’s him regaining his colour after using a disguise. We’ve seen him running through all these realms, and for all we know, he needed to wear a disguise through them. Maybe he even disguised himself as Chin Chin so people wouldn’t get as suspicious when they saw Pink Guy running around these densely populated realms.
Afterwards, he puts back on his real colour. I think Pink Thug is another disguise Pink Guy wore, hence him being covered in the pink liquid at the end too.