I know about *bold*, _underline_, etc. which can be used to highlight text. This is shown in the text itself and in the export.
Is there any other way to highlight text *within* Emacs? I'm looking for something similar to how org-mode highlights the TODO word. I would like to, for example, highlight two lines in a plain list in one colour and another two in a different colour. Is this possible? Or is this more of an Emacs question rather than an org question?