A combination of two first-order reactions that had different rate constants was found to describe the rate of color development in the 288-343 degrees C diesel fraction from hydrocracked Llyodminster atmospheric bottoms. These two reactions were the conversion of color precursors which were either (a) unsaturated compounds or (b) heteroatom compounds. The color precursors were correlated with the composition of the diesel fraction. The heteroatom color precursors were correlated with the combination of sulfur and nitrogen content of the diesel fraction. The unsaturated color precursors were correlated with its density, since density can be related to the aromatic content. The rate constants were related to catalyst composition. The fact that all of the parameters in the correlation were related to physical phenomena supported the hypothesis that color development is related to these two types of color precursors. The hydrocracking conditions were 17.3 MPa, 415 degrees C, 0.59 LHSV, and 570 L (Hz at STP)/L of feed [2400 scf/bbl]. Eleven different hydrocracking catalysts were examined, most of which contained two components, a conventional CoMo:gamma Al2O3 component and a zeolite component (sodalite containing nickel, SOD-Ni). The same two rate expressions were found to describe the results obtained from all of the catalysts. The conversion of the heteroatom compounds that were color precursors was associated with the CoMo:gamma Al2O3 component of the catalyst component. The conversion of the unsaturated compounds that were color precursors was associated with the SOD-Ni catalyst component.