Purple, but not the one that's sold as purple, which isn't a very nice colour, instead mix equal quantities of cobalt blue and fluorescent pink, and it makes an absolutely stunning proper purple.
We first discovered this when we filled Maria's dress with alternating blue and pink gunge, which resulted in an absolutely gorgeous purple goo dripping and flowing out from inside her dress.
Personally, I like the white like you used in your "glue" sets. Kinda ties in nicely as a punishment too, if you want to add a slightly dramatic aspect to the vids.