2nd part of Dileep Sankhlas GSoC 18 project.
The original work is in D13203,Changing typewriter text color can be done in the typewriter properties dialog, or programmatically via new okular API methods TextAnnotation::textColor and TextAnnotation::setTextColor.
poppler >= 0.69 is required to store text color natively inside PDF documents. For other document types, text color is stored as metadata inside the document archive.
This work was done during GSoC 2018. Diff ID range 37290 to 38049See https://community.kde.org/GSoC/2018/StatusReports/DileepSankhla for details.