The purpose of the work is to explore the feasibility of devising a computer system that implements the desirable effects of a photographic filter and provides an environment for colour filter design for image processing. Using conversion from RGB to the CIELUV colour space a new method for the implementation of photographic filter as a digital filter is described. A filter is implemented by converting image pixel rgb values into CIELUV (u', v') and L* values and operates using the visual wavelength values provided by the (u', v') chromaticity diagram. However, the (u', v') diagram cannot provide wavelength values for pixels that correspond to (u', v') points in the `purple line' sector of the diagram. These pixels are allocated wavelengths by means of a new wavelengths scale that makes it possible for the filter to process any pixel in a digital image. Filter transmittance data for visual spectrum wavelengths is obtained from published tables. The transmittance data for purple sector pixels is provided by a colour model of the (u', v') chromaticity diagram. The system is evaluated by means of the Macbeth ColorChecker chart and the use of physical measurements. The extension of the CIELUV diagram with an equivalent wavelength scale provides a new environment for the enhancement and manipulation of digital colour images.