It shows two colors while it does not have two colors.
The second color can be seen while you use an incandescent light.
For instance the gemstone will change from green to red once is put under the incandescent light.
Nowadays we have a mix-up concerning the color change gemstones, multicolor gemstones and bicolor gemstones. Especially when handling the rough gemstones.
For you to see the color change on the gemstone you must have two types of light sources. That is “fluorescent light” and “incandescent light”.
This one can be obtained by getting fluorescent bulb and even the sunlight can do the work just as good as the fluorescent bulb.
This can be obtained by using an incandescent lamp which has incandescent bulb or candle.
The principals are the same, even if it is a synthetic sapphire. The same principals are applicable for all gemstones. Just take a gemstone that shows the color change, then put it under the fluorescent lamp, take for instance sapphire.
To see the second color, take the same gemstone put it under the incandescent lamp.
Then you will see the second color.
By this procedure the sapphire which appeared green under fluorescent light will change to red under incandescent light.
And once taken away from the incandescent light, it will return to green again.
The multicolor gemstones are not the change color gemstones, such as tourmalines is the multicolor and not the change color. Especially sapphire gotten from Tanga region here in Tanzania has more than one color. And this does not make this type of sapphire to be the change color, famously known as “Umba sapphire”.
The amazing thing is the colors can be seen while under the fluorescent lamp only, this confirms to you that it’s a multicolored sapphire or a bicolored sapphire NOT change color.
Here are some color change gemstones
Take note since you are in the gemstone field, it is good you have those two types of torch or lamp. That is fluorescent lamp/torch and incandescent lamp/torch.