1. A method comprising: determining a first inner set of points along an inner perimeter of a source region within a first image, wherein the source region includes a source-center-point; for each point in the first inner set of points, assigning an inner color to the point based on one or more inner colors in the first image at the point; determining an outer set of points along an outer perimeter of a target region within a second image; for each point in the outer set of points, assigning an outer color to the point based on one or more outer colors in the second image at the point; wherein each point in the first inner set of points has a corresponding point in the outer set of points; producing a set of difference colors by determining a difference color between each point in the first inner set of points and the corresponding point in the outer set of points; determining a second inner set of points along the inner perimeter, wherein each point in the second inner set of points is within a region based on the first inner set of points; producing a set of blurred difference colors by determining a blurred difference color based, at least in part, on the set of difference colors; wherein each point in the second inner set of points corresponds to a particular blurred difference color in the set of blurred difference colors; generating a color map comprising the set of difference colors and the set of blurred difference colors; updating each color within the target region based, at least in part, on a corresponding source region color in the source region and the color map; wherein the method is performed by one or more computing devices.