1. A method of calculating a noise characteristic value for a source digital image, comprising the steps of: a) receiving a source digital image including pixels corresponding to two or more different colors; b) using pixel values corresponding to a first color to produce at least one color correlation term for pixels corresponding to a second color which respectively indicate the extent of the noise content in the pixel values of the second color; and c) using the color correlation term to calculate a noise characteristic value for the pixels corresponding to the second color.