Archaeologists studying rock paintings in Australia and elsewhere have commented on differences in the colors of paint used through time, notably the apparent dominance of reds over yellows in early paintings. A study of rock paintings near Cloncurry, northwestern Queensland, Australia, has revealed that goethite, a yellow to brown mineral, is being converted to hematite, a red mineral. The implications for chronological interpretations could be significant, especially those concerned with color superposition. The effects of temperature, grain size, and relative humidity on the conversion of goethite to hematite are reviewed. Wetting and drying of goethite will favor hematite production. The paintings in the area studied are commonly red, but some sites also have yellow paint. One figure reveals three different colors. It appears that physical processes, resulting from differences in exposure to sunlight (heating) and groundwater flow, have modified one original monochrome pigment. Some fragments from an ocher quarry were found to be two-colored, red and yellow, with the red portion upturned and exposed to the elements. Analysis of small fragments from paintings in the region, and from the quarry, by x-ray diffraction (XRD), scanning electron microscopy (SEM), and petrographic examination, showed that rounded goethite-rich ocher fragments have undergone a partial transformation to hematite in those portions exposed to the weather. Whether pigment was applied to the rock as crayons or as wet paint consisting of ground ocher particles, the resulting reduction in grain size further enhances the goethite-hematite transformation. Perhaps yellow paint only survives in environmentally favorable conditions.