The invention provides a method of assessing the potential colour protecting effect in vivo of a test formulation such as rinse off hair treatment formulation the method comprising the steps of: providing a sample of the test formulation to be assessed providing a model substrate in the form of a hair switch which has been artificially coloured applying the test formulation to the hair switch subsequently rinsing the test formulation from the hair switch collecting the rinse liquor and analysing it for the presence of eluted colour characterised in that the artificial colour used to colour the hair switch is a permanent hair colorant with a shade level 1 or 2 according to the International Colour Chart (I.C.C.). The method provides improved sensitivity and differentiation and enables the tester to compare different technologies in terms of effectiveness.
