Posca paint pens, in various widths. These are perfect for label writing.

1M 0.7-1mm extra fine tip

1MR 0.7mm extra fine tip

3M 0.9-1.3mm fine bullet tip

5M 1.5-1.8 mm medium bullet tip

Water based paint. 

To activate your pen, shake thoroughly and then press down to release the paint.

If the tip is dry or dirty, remove it and soak in water. 1MR is not removable, but can be soaked if it's dry or dirty.

The PC-3M and PC-5M tips are reversible, which means you can flip them over and double their longevity!