The invention deals with a method for replacing the colorant values of a raster image by a new set of colorant values that is for example optimized for a standardized printing process. According to the invention, a forward look up table is identified that transforms the colorant values in the raster image into color values. The color values in this look up table are separated into new colorant values to obtain a colorant to colorant interpolation look up table. This is done by using an inverse transformation that is based on the forward look up table using appropriate separation parameters for the standardized printing process. The method has the advantage that it enables to map pure colorant values in the original image back onto pure colorant values in the transformed image.