Hi,
Sorry for keeping you waiting.
Based on my research and test, we may take the following steps to change the default reply text color in Outlook 2013:
1. Go to File > Options > Mail > Stationery and Fonts, navigate to the Personal Stationery tab.
2. Select Use my font when replying and forwarding messages (You might need to choose a Theme to modify this option, then select No theme if you do not need a theme for new HTML e-mail message.).
3. Click the Font button, choose black for replying and forwarding messages:

For more information about how to configure the default font color for replying and forwarding messages in Microsoft Outlook 2013, you may also refer to the link below:
https://www.extendoffice.com/documents/outlook/1468-outlook-reply-font-size-color.html
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Hope you can find this helpful.
Best regards,
Yuki Sun
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