Sweetheart used to be part of the Crisp Formula collection where the inks were designed to work well on a variety of papers. Birmingham has since recategorized their inks and now this can be found under the Keystone Formula collection. This ink is a vivid pink, almost fluorescent, with a tinge of orange that comes out in the wetter areas.
The ink has an average flow. On Muji paper, there is shading, no sheen, no feathering and no bleedthrough.
This is written with a TWSBI ECO-T medium nib.
Below are a couple of photos of how it looks when it is written on both sides of the paper.
Disclaimer: This ink was purchased by me and all opinions/photos are my own. This post was not sponsored.