Paytm Mall - Refusing to Replace My Item

I had ordered for a "Seagate *TB Backup Plus Slim Portable External Hard Drive (Grey)" from Paytm Mall on *th August, **** and it was delivered on **th August, **** as per the promised date. The product ordered was Grey in colour and upon opening the package, I received a Pink Hard Disk. I immediately called Paytm Care on ****-******* and registered a complain. They said that a refund wasn't possible, but a replacement would be approved on submission of evidence. I sent them a picture of the wrong colour of the product, of the product barcode & serial number, of the product box and of the Paytm Mall outer box as well. My replacement was approved that same night and the courier was scheduled for a pickup the next day (Monday, **th August). When the courier came, he refused to take my product back as the description in his email said that it was Grey in colour and I was handing him a Pink Hard Disk. He simply rejected the request and left. I instantly called Paytm Care and brought this issue to their notice and they mentioned that they're setting up a new pickup request and this time, they will mention that the product colour was wrong. By around *:**pm on Monday, **th August, I got a request approved email from their side. But again, at **pm the same day, I got an email saying the return is rejected. I have constantly been calling them since Monday night and they're always asking for ** hours, but no one is providing me any resolution. They are highly incompetent and DO NOT RESPOND properly at all. I'd really appreciate with some help in dealing with this.