Observers with a colour vision deficiency usually find it more difficult to discriminate between red and green colours, due to genetic variation in cone spectral sensitivity. Daltonization methods aim to enhance colour stimuli in order to increase the visual difference between them for such observers. In this work we focus on filtering the stimulus in the reflectance domain prior to conversion to colorimetry. This is hypothesized to enable a more precise tuning of the enhancement to the spectral absorptions of the observer with a colour vision deficiency. A spectral sharpening filter was developed and applied to spectral reflectances for a range of colour pairs which observers with a colour vision deficiency would be expected to find difficult to discriminate. The reflectance pairs were converted to colorimetry and presented on a colour display, where the visual difference between the pairs was evaluated by observers in a psychophysical experiment. Results suggest that a suitable filter can lead to an increase in the difference between the red-green pairs.