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. See https://community.kde.org/GSoC/2018/StatusReports/DileepSankhla for details.