This is quite straightforward to set up. To do this follow these steps:
On the history of a case select Choose columns at the bottom of the screen.
On the right-hand side choose the action type you want to change the colour for;
On the selected items, select the top row and hold shift and select the bottom row.
Select a colour from the pallet at the top right.
Click save.
You have now set up colour coded history lines.