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my blue-eyed siamese-mix cat has great blue eyes. They seem to darken appreciably at certain times of the day, and maybe with his 'moods'. Is this my imagination?
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I find it quite possible that the eye colour can slightly change depending on the light conditions. The same happens to the human eye as well.
Comments  
Are you referring to the pupils dilating when it gets dark? Cat's pupils dilate in lower light and also depends on their mood sometimes if they are being very playful they might dilate which would make the eye look darker.
My cat too, it change its eyes's color from blue to red, fastly!! In a certain time. When its eye was blue, it act like normal. But when it changed to red. It became extremely agressive. It was not affected by the light cuz i had seen this thing in many ways of places and times. Is it normal?? Cuz sometimes i scared too.