Organic cotton and unbleached cotton color changes over time
All cotton will change as it ages. Cotton that is organic and unbleached will get lighter over time. Chlorine or oxygen bleach will lighten it. Detergent ingredients will lighten it. Sunning will lighten it. Chlorinated municipal water will lighten it.
That's ok. It's normal.
Here is an old diaper that has been sunned and a new one. They are tagged correctly, but cotton really does change with washing and use.