1. A printed dynamic optical illusion image printed by a printing device on a print media using a plurality of colorants, wherein one or more of the colorants are appearance mutable colorants having spectral characteristics that can be switched between a first colorant state and a second colorant state by application of an appropriate external stimulus, and wherein one or more mutable portions of the printed dynamic optical illusion image are printed using at least one appearance mutable colorant; wherein when the mutable portions are in a first appearance state the printed dynamic optical illusion image has a first illusion state, and when the mutable portions are in a second appearance state the printed dynamic optical illusion image has a second illusion state, such that changing the optical illusion image from the first illusion state to the second illusion state affects the perception of an optical illusion by a human observer; the printed dynamic optical illusion image being switchable between the first and second illusion states by applying the appropriate external stimulus to switch the one or more appearance mutable colorants between their first and second colorant states, thereby switching the mutable portions of the printed dynamic optical illusion image between their corresponding first and second appearance states.