Aspects of the present invention provide machine colour characterisation for augmented reality. A process of machine colour characterisation for digital images comprises generating colour model data for each of a set of colour categories dependent on colour captured in example digital media from example colours in an example image, the colour model data for each colour category defining a constraint in a colour space. The constraint is defined dependent on a set of example pixels of the example digital media, and the process comprising relating pixels of the image to be characterised to the constraint of one or more colour models to identify colour categories for the pixels of the image to be characterized to characterise the digital image. A transformation is computed for a mean of the example pixels expressed using colour components to the origin of the R,G, B cube and transforms one or more axes determined for the pixels to align with R,G or B axes. A constraint may be defined as a first bounding volume. The bounding volume may be a bounding volume corresponding to ranges of example pixels of one example colour in the R, G, B cube.