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I understand that you facing issues with printing in grayscale from your HP OfficeJet Pro 8715 All-in-One Printer.
This issue could occur if the print settings and not done properly.
Let's go through a few steps to resolve this issue.
1. In your software application, click Print.
2. Select your printer from the list of available printers.
3. Click the option that opens the Printer Properties dialog box. Depending on your software application, this button might be called Properties, Options, Printer Setup, Printer Properties, Printer, or Preferences.
4. On the properties window, click the Paper/Quality or Features tab, depending on your software.
5. Click Advanced, click Print in Grayscale, and then select Print in Grayscale or High-Quality Grayscale.
6. Click OK, and then click Print to start the print job.