By looking at things our visual cortex perceives as blue, typically by activating the blue photoreceptors more than others relative to objects that are not blue

By looking at things our visual cortex perceives as blue, typically by activating the blue photoreceptors more than others relative to objects that are not blue but also via negative after images and a number of other methods. Look at these blue hearts, for example: https://cdn.cpnscdn.com/static/blog/2015/02/greenblueheartsstriped.jpg https://cdn.cpnscdn.com/static/blog/2015/02/greenblueheartss... You seem to be making the mistake I specifically warned against of defining color by the distribution of wavelengths coming from the object. Once again, the definition of color is ambiguous, and most people will use the human perception definition. Purple things don't have to be composed of pure violet light.