No one's eyes change colour (Bar babies to adults) the colours are genetically unique and much like a tiger's stripes, in regards to having a consistent pattern. Thus the reason iris tracking is being suggested in many security branches. Hazel eyes have the fortunate trait of having both green (the most common colour) and brown shards of colour, which as physicists may know, reflect hugely different proportions of visible light. This leads to varying appearances based on light levels.