As per the description above, you want to change the color of the text in Fill&Sign, Is that correct?
The text in Fill & Sign is only black, there are no color options available to change the text color as it is a designed behavior. For detailed information, you can refer to Adobe Sign FAQs
However, if you wish to file a feature request, you can use the URL Feature Request/Bug Report Form
Feel free to update this discussion for any further assistance.