This could be due do a few different issues.
Your plant may be over or under watered. Make sure the soil is moist but never soggy. Make sure your container has adequate drainage holes in the bottom and that they are draining.
Often a container that sits directly on ground may not drain freely. You can sit the container on a few bricks to raise it off the ground.
Nutritional issues in the soil could also cause the color changes in the leaves.
I like to give my Herbs Epsom Salt.
Here are some links with more information.
https://www.gardeningknowhow.com/edible/herbs/mint/catmint-plant.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/calcium-fertilizer.htm
https://www.gardeningknowhow.com/plant-problems/environmental/leaves-turning-reddish-purple.htm
