The core of a colored pencil is the solid, colored material running through the center of the pencil, surrounded by a wooden or synthetic barrel.
Core is the technically correct term for the colored material inside a pencil, although it is sometimes informally called the lead. Colored pencils do not contain graphite or lead.
The core is made from a mixture of pigments, binders, fillers, and other ingredients. Its formula determines the pencil's characteristics, including firmness, glide, color payoff, adhesion, and point retention.
Core diameter varies between pencil brands and product lines. A wider core contains more coloring material but is not necessarily softer, more pigmented, or higher quality than a narrower core.
In the Lab, I don’t measure for core diameter. Read why here.