This invention relates to improved imaging systems based on the formation of yellow colored coordination compounds of transition metals with certain ligands. These coordination compounds have been found to provide excellent yellow colors when used in pressure sensitive carbonless copy-papers wherein the image is formed by the reaction of a color-forming compound with transition metal salts such as those of nickel, cobalt, iron, copper, and similar materials. These yellow color-formers have the advantage of high solubility in encapsulation solvents and have less color on Zn.sup.2+ containing CB sheets. Use of these yellow color-formers with other metal complex color-formers such as N-(monosubstituted)dithiooxamide color-formers and N,N'-(disubstituted)dithiooxamides results in the formation of black images.