Color 24
Color 24 is another special color number within the set of LDraw colors, in addition to the special Color 16.
Color 24 has a different purpose than color 16: whereas color 16 is being used by part surfaces (quads, triangles, subparts etc.), color 24 is only intended to be used by edges.
It does not hardcode a specific color but instead carries the semantics "let software that operates on this file choose a color for the edges that looks best with the part's color".
For this reason, all official parts have all their edges colored in color 24 (even the conditional lines).
This way, software can pick a "good" color for the edges that goes well with the color a part finally has. That color is not known beforehand, because usually official parts carry the color 16, meaning that the color is to be chosen by the end user.
Color 16 and color 24 are one of the fundamental inventions of James Jessiman when creating LDraw. This simple concept is what makes the LDraw library so versatile. It is one of the core key concepts of LDraw.