To add these, you need to access the CSS editor an invoice. This can be done by viewing an invoice, and then accessing the invoice gallery:
And then clicking 'Customise this template':
invLabel_05 (which, to confirm, is the "Due Date" label), you would add something like this to the CSS editor once it loads up on the right hand side:
When you save this, it will confirm and then refresh the page. At this point, the label should be changed for you.
#ff0000 equates to red, so removing both pieces that you've pasted, would simply remove the red from the "Due Date" line on your invoice.
These changes would apply to all invoices.
I hope that helps, but please let me know if we can help further