Too low of light
The likely cause of your Dragon’s Tongue leaves losing their purple coloring is due to lack of light. Dragon’s Tongue plants prefer to live in a spot with bright indirect light. They can adapt to lower light, but that vibrant, purple coloring will begin to fade.
If you’d like to ensure the leaves stay purple, make sure it is getting enough light. Bright indirect light can be found in places next to an east-facing window or a few feet back from an unobstructed southern or western window. If the southern or western window has something like a sheer curtain, natural shade from a tree or building outside, the plant can be placed a little closer. To learn more about indoor plant lighting, check out our lighting guide here!