1. Apparatus for processing image data comprising storage means for storing instructions, memory means for storing said instructions during execution and for storing image data, processing means for performing image processing in which said image data is processed to modify color values, and display means for facilitating user interaction with said image processing, wherein said processing means is configured such that, in response to said instructions, said image data is processed by the steps of: identifying a color vector and a luminance range for said color vector; defining a color vector function in response to said identification, in which said color vector is a function of luminance; processing source image data to identify luminance values; and modifying colors in response to said luminance values with reference to said color vector function.