Sorry for the delayed response.
As per the issue description mentioned above, you want to know how to change font color under fill & sign, is that correct?
Sorry to say but the option to change font color under fill & sign is not available. You may refer to the following forum threads which discuss the similar topic:
Let us know if you have a further query.
The feature you are looking for is/was part of Adobe reader in version 11, if you downgrade to that version you can change the font color.