Use this Uni-ball One gel ink refill in the pens listed under Compatible Products.
Its ink is specially formulated with large pigment particles that stay on the surface of the paper instead of getting absorbed into it.
Since the ink isn't getting washed out by the paper, dark inks look darker, and bright inks look brighter.
The ink is fast drying and bleed resistant too, so your notes will always be crisp and free of smudges.
Type: Refill
Nib Size: 0.38 mm, 0.5 mm
Diameter: 6.0 mm
Ink Composition: Pigment-Based, Water-Based
Size: 11.2 cm
Standard Refill Style: Euro Style
Tip Material: Metal
Tip Type: Conical
Water-Resistant: Yes
Weight: 3 grams
Brand: Uni
Manufacturer: Mitsubishi Pencil Co. Ltd., Higashidai Shinagawaku Tokyo