What makes highlighters have that "neon" color??

Speckloid Oxide added to the color pigment it is a type of chemical that adds fleurecent (Neon) Effects to colors (I.E. Add it to lime green pigmentation to make a neon green). Hope this helped!!

Many highlighters come in bright, often fluorescent colours. Being fluorescent, highlighter ink will glow under a black light The most common color for highlighters is yellow, but they are also found in pink, blue, green, orange, and purple varieties. These highlighters flourese, displaying a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength. The energy difference between the absorbed and emitted photons ends up as molecular vibrations or heat. Usually the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral fluorite, composed of calcium fluoride, which often exhibits this phenomenon. A highlighter is a phosphorescent and fluorescent marker, comprising both high and low molecular weight polyethylene glycols in combination with phosphorescent and fluorescent pigments. The composition advantageously preferably further includes at least one filler, which preferably includes a water-miscible filler,