Refilling a chalk line is not a complex process, but if done incorrectly it can be a messy one.
Most importantly in the process of refilling make sure the surface is flat and there are no distractions. Unscrew both side screws and firmly but carefully, pull the top casing off. Add chalk to the box until it reaches the fill line. Replace the top lid and screws. Shake the box a few times to distribute the chalk.
Any chalk residue should be blown off, not wiped, to avoid smearing.
What Color To Use?
White chalk is the best choice for interior use because it is the least permanent. White chalk won't bleed through wallpaper or the paint and blows off easily.
Blue chalk works best on outdoor surfaces where a balance between resistance from the rain and removability are desired.
Red chalk is the most permanent. Use red chalk outside where it needs to withstand rain, wind, or abuse. Red is an ideal color for using on the ground, on tarpaper, on in places where the line will be covered up eventually.
Neon chalks are the most visible and designed to be used on grass or other heavily textured surfaces where a bold line is necessary.