An image rendering apparatus includes processing circuitry to obtain a volumetric medical imaging data set representative of a region of a material; associate one particular extinction color with the material; determine, based on the particular extinction color, a chromatic attenuation profile representative of color value as a function of intensity and as a function of depth in the material, the chromatic attenuation profile showing how the material having the particular extinction color changes in appearance with changes in the depth; obtain, from a user, a color value; determine a modified extinction color for the material based on the color value obtained from the user; and render a global illumination image from the volumetric medical imaging data set using the modified extinction color.