Use for inking to trace hollow lines. It g gives the model a three-dimensional feeling and makes it appear more realistic. The effect is stronger when you choose the color to ink according to the color of the model parts. In general, you can use black for dark parts, grey for lighter parts and brown for red parts.
Gundam Marker: Black (Fine-tip for panel lines) (Renewal)
Gundam Markers 3 Black, Brown & Grey extra Fine Tip / for Panel Lines Uk
GSI Creos Mr. Hobby GM01 Black Fine Point Gundam Panel Line Marker
GSI Creos Black Fine Point Gundam Marker GM01 – USA Gundam Store
Black Color, perfect for most colored surfaces Small sized tip for most normal sized panels From the makers of Mr. Hobby/Mr. Color paints
GSI Creos Black Fine Point Gundam Marker
Detail Pens & Markers - Paint, Decals, Mask & Accessories
The metallic markers are so worth it! : r/Gunpla
Bandai Gundam Marker Fine Line (Black) [BANGM01] - HobbyTown
Gundam Marker Pour-Type Panel Liner (Select - Black, Brown, or Gray) – Gundam Shoppers Network
Gundam Marker Black Fine Line
GSI Creos Gundam Marker Pouring Inking Pen Set Detail Builders Parts GMS122