If you're being headshotted/exploded/killed - Jar immediately explodes into a jarate fluid at big radius, as it gives Jarate effect, mini-crits.
The colors that are gonna affect to effect taken: White, Green, Yellow
If this cosmetic has been painted by other colors that are not included on above, the cosmetic-weapon won't work.
White Paint: It will be just like a Mad Milk.
Green/Yellow Paint: Jarate effect, definitely.