1. A method, comprising: performing, by a computer: receiving input initiating a non-uniform paint loading mode for a virtual brush; and subsequent to receiving the input initiating the non-uniform paint loading mode and while in the non-uniform paint loading mode: receiving input selecting a region of an image on a digital canvas having virtual paint that is applied in a non-uniform distribution of paint colors; and performing a non-uniform paint loading operation with the virtual brush, including loading the virtual brush with virtual paint having a distribution of colors that corresponds to the non-uniform distribution of paint colors in the selected region without affecting said virtual paint applied to the selected region, the loading the virtual brush with virtual paint including: obtaining two or more samples each comprising values of each of a plurality of canvas pixels under the virtual brush as the virtual brush is swept over the selected region; and storing data in each of a plurality of cells of a brush model for the virtual brush representing a paint color in a respective portion of the selected region, the storing including, for each sample taken, updating data in one or more cells of the brush model to reflect an addition of virtual paint of the paint color in a respective portion of the sample.