A computer-implemented method for evaluating which of a set of different types of reference indicator has colour attributes closest to those of an object is disclosed herein. Each respective type of reference indicator has different colour attributes. The method comprises determining a colour relationship between the object and at least one reference indicator of a first type by analysing a plurality of image portions, each image portion comprising at least part of the object and at least one reference indicator of the first type placed directly on the object. The image portions each have different lighting characteristics. The method also comprises comparing the determined colour relationship to known colour relationships associated with the at least one reference indicator, each known colour relationship representing a relationship between the reference indicator and another reference indicator of the set of different types of reference indicator, to identify which of the set of different types of reference indicator has colour attributes closest to those of the object.