Qt QPlainTextEdit background

Slightly confusingly they call it role rather than colour/color.

https://doc.qt.io/qt-5/qwidget.html#setBackgroundRole

hint - if you can't find a function for a particular control, click on show inherited members - most general settings are in qWidget which is the basis for eveything drawn on screen.

If QPlainTextEdit supports style sheets, you could do it like this:

myPlainTextEdit->setStyleSheet("background-color: yellow");

or

qApp->setStyleSheet("QPlainTextEdit {background-color: yellow}");

Modify the palette of your plain text edit. Sample program:

#include <QApplication>
#include <QPlainTextEdit>

int main(int argc, char* argv[])
{
  QApplication app(argc, argv);

  QPlainTextEdit edit;
  QPalette p = edit.palette();

  p.setColor(QPalette::Active, QPalette::Base, Qt::red);
  p.setColor(QPalette::Inactive, QPalette::Base, Qt::red);

  edit.setPalette(p);

  edit.show();
  return app.exec();
}

Substitute whatever color you want, of course.