Hi, black skin on cheeks or gums may be because of some injury/ constant irritation/ habits like smoking, or tobacco chewing also cause pigmentation. Coming to your case, you said you are using Coolip, if that's the case then how long are you using Coolip? Did you observe this black-and-white discoloration just a few days after using it or was it there before coolip usage? If possible share a picture of that area for better diagnosis.
Next Steps
Visit a nearby dentist, Dr will check and confirm
Health Tips
Though it is not painful or not bothering you there is no harm in visiting a Dr for a checkup and clearing your doubts. Sometimes our body gives alerts like this so always prevention is better than cure.