Two major factors causing a change in colour matches between large and small fields are macular pigmentation and cone outer-segment lengthening We have simulated the effect of the macular pigment optical density (o.d.) and of the cone effective o,d. on colour matches and demonstrated that ignoring the change of cone o.d. between the fovea and the periphery leads to over-estimation of the variation of macular pigment o.d. We have examined the colour matches of 4 observers on 30 degrees, 7 degrees, 3 degrees and 1.2 degrees fields, published by Vienot (1983) and modeled the change of macular pigment o.d and cone o.d, with eccentricity. Neither macular pigment o,d. nor cone o.d vary much beyond 7 degrees diameter. For the observers, the difference of macular pigment o.d. between 2 degrees and 10 degrees fields is within the range 0.22-0.41 and the difference of cone o.d. between 2 degrees and 10 degrees fields is within the range 0.07-0.12, when the change of cone o.d is correctly allowed for. (C) 2000 John Wiley & Sons, Inc.