A nonmagnetic monocomponent negatively chargeable color developer which has excellent light transmittancy for overhead projection transparencies, excellent transfer characteristics, charging characteristics, and durability is provided. The nonmagnetic monocomponent negatively chargeable color developer contains color toner particles that contain a chromatic color colorant, binder resin, and charge control agent, and an exterior additive added to the exterior of said toner particles. The binder resin is a linear polyester resin having a softening point of about 100.degree. to about 120.degree. C., the sum of the acid value and the hydroxyl value is about 20 to about 40 KOH mg/g, and the weight-average molecular weight/number-average molecular weight ratio is less than about 10. The charge controller is a boron compound having the general structural formula (A) below: ##STR1## wherein R.sub.1 and R.sub.8 is a substituted or nonsubstituted aryl groups; R.sub.2 and R.sub.4 is a member selected from the group consisting of hydrogen, alkyl group, substituted or nonsubstituted aryl group; X represents a cation, and n is an integer of either 1 or 2. The exterior additive is a hydrophobic silica and hydrophobic titanium dioxide in a hydrophobic silica. The hydrophobic titanium dioxide weight ratio is about 1:1 to about 1:3 by weight. The total specific surface area S of said exterior additive microparticles is about 100 to about 300 (m.sup.2 /g.multidot.wt %), and is determined by equation (1):S=(Ss.multidot.Vs)+(St.multidot.Vt) (1)wherein Ss is the specific surface area (m.sup.2 /g) of the hydrophobic silica; Vs is the additive amount (weight percent) of hydrophobic silica relative to the toner particle; St is the specific surface area (m.sup.2 /g) of the titanium dioxide; and Vt is the additive amount (weight percent) of titanium dioxide relative to the toner particle. The total amount of added exterior additive microparticles is about 0.8 to about 2.5 percent-by-weight relative to the toner particles.