You can assign a colour to your notes and establish your own colour code.

From the message list

By clicking on the button at the very bottom left side of the message : 

A colour list appears, you only have to pick a colour (or change it).

When sending your note

From the messaging area, pick a colour before sending your message :

From the notes management

Click on the colour icon to display the colour palette :

Note : this process also works for tasks.