There is nothing wrong with the plant losing it’s purple color and is normal on older leaves that will die off. There isn’t much you can do to make it more purple. Its highly unusual to attempt to, but exposing it to brighter light may help. It will sunburn and bleach if given too much, so just a little more light and not too much. Stress may help it color up more too. You can do this by not fertilizing it and let it completely dry between watering. Let it go fairly dry, but not too dry that it damages leaves. Just enough lack of water that you can tell it’s beginning to wilt, but make sure that when you do water that the water goes all the way through the drainage holes of the container. This type of stress will help it color up. This is a bit unusual though and not a common practice.