It should be green in sunlight and fluorescent light.
It should be purple or reddish-Purple in soft incandescent light. A low-lit room is the best place to see the color change. Light from a candle will also show the purple color change. Very, very infrequently an alexandrite will exhibit a change to a more reddish color- reddish-Violet, Violet-red etc. These gems command a great premium in price.
Mixed light will produce either a grayer color as the green and purple mix or flashes of the green, purple, and sometimes red or yellow. Strong color-change stones will usually show the red/purple color in mixed lighting
Rotate the stone to get the color from all facets and to look for 'flash' (blazes of color that show in the body of the stone). Ruby-red and hot-Pink flash and sparkle is the most desirable and most rare.
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Swiss Gem Shop loose Gemstone at Sale price and Gemstone Information