Skin colour reflects the pathological condition beneath the skin and is commonly used to indicate extent of a lesion. Psoriasis is a skin disease that is indicated by the appearance of red plaques. Although there is no cure for psoriasis, many treatment modalities exist that can help control the disease. To evaluate treatment efficacy, the Psoriasis Area and Severity Index (PASI) as a gold standard method is used to measure psoriasis severity. Commonly, the erythema (a PASI parameter) of a lesion is assessed visually but may lead to subjective and inconsistent results. Since psoriasis lesions can have a wide variety of colour, modeling the psoriasis colour is necessary. In this work, we proposed a colour model of psoriasis lesion that can be used in PASI erythema scoring. The colour of psoriasis lesion is modeled by hue (H), saturation (S), and value (V). Correlation coefficient between HSV parameters of lesion references with PASI erythema score is calculated. Among the three parameters, hue and saturation are the two most correlated parameters with PASI erythema scores. Larger numbers of lesion references are required in order to model the distribution of lesions with different scores in hue-saturation plane. © 2008 IEEE.