Black color ink. The FitCurve is a comfortable and high-quality ballpoint pen from Kokuyo that was requested many times from our customer because of its natural grip, balanced body weight, pearlescent body sheen and smooth writing. Based on ergonomic research, the center of gravity of the pen is located near the natural grip location, reducing \"hand wobbling\" that can cause fatigue. The body shaft measures 10 mm in diameter, thinner than standard ergonomic pens to account for how the pen is used when at a tilt. Apparently, when pens are held at a 30° tilt, 1.15 times the diameter is experienced in the hand. Therefore, the FitCurve is designed thinner so that it can have a 11.5 mm diameter feel when in use. Ink refills available.