Since transitioning to work, but still keeping a notebook, I use the following. Colours based entirely on what pencils/highlighters I had when I first started taking notes at work.
<span style="color:#323232;">Assigned work Blue
</span><span style="color:#323232;">Ad hoc Orange
</span><span style="color:#323232;">Errors Pink/Red
</span><span style="color:#323232;">Solutions Green
</span><span style="color:#323232;">Actualizing Yellow
</span><span style="color:#323232;">WTF Purple
</span>