We're committed to developing an easy to use financial tool that will help you today and grow with you tomorrow. At this time, you're unable to modify font color or size in QuickBooks Online. However, I'll send feedback to our Product Developers so they can consider a display preferences section. The Product Development Team regularly reviews customer submitted feedback. You can always look into our Blog to check and see what's new in the upcoming releases.
If you're referring to invoices, estimates, or sales receipts, then I recommend customizing them. This will help by making your form designs look more professional to your clients/customers. Also, to help in managing your business, here's our tutorial page. This Community based page has videos and additional details to help you navigate through your QuickBooks.
If you need any further assistance, please don't hesitate to ask. I'm always here to lend a helping hand. Wishing you continued success.