include header and replaced glColor3ubv
You will need to replace glRecti which is where the "fun" begins (you will need to bind a program, pass a vertex format, ...).
Also @Fable Fox (fablefox) please edit the wiki to claim this file for you: https://wiki.blender.org/index.php/Dev:2.8/Source/OpenGL/Immediate_Mode_Replacement_Tasks
I have to leave this OpenGL replacement job as-is due to work and also I'm taking OCA 12c exam end of this month (also related to day work). Glad to see it progress well on the task page.
Could use the UNIFORM_COLOR shader here.