1. A method of predicting colours resulting from using one ink on an ink jet printer, said method comprising the steps of: producing a test page for said printer, said test page comprising a plurality of sample ink patches, each said patch comprising a plurality of ink dots with each ink dot being of a predetermined quantity of ink; measuring each of said sample ink patches to produce a plurality of colour space coordinates for each said patch; and fitting values representing a total quantity of ink for each said patch and said colour space coordinates to a plurality of predetermined functions using a minimisation process, wherein said predetermined functions are of the form P x (1 −f x ( a ))&plus; I x f x ( a ); P y (1 −f y ( a ))&plus; I y f y ( a ); P z (1 −f z ( a ))&plus; I z f z ( a ); where a represents said predetermined quantity of ink, f x (a), f y (a) and f z (a) are functions of a, and wherein P x, P y and P z are colour space components representing a paper colour, and I x, I y and I z are colour space components representing the ink colour.