First off, welcome to the Community, and thanks for reaching out with your question.
In the Desktop version of QuickBooks, you have the ability to change the font color for any specific text box you have set up on the invoice. You'd need to go into your invoice template, double-click the text box you'd like to change, and select the color you like.
For QuickBooks Online, you're only able to change the heading on the invoice, the font color is not able to be edited.
Don't hesitate to reach back out, if you have any other questions. Thanks.