Under Settings => Organisation, you can change how the colours are assigned to tasks. Tasks can be colour coded according to Client, Task, Job, Manager, or Assigned To. Choose your option and click Save Organisation Settings.
If you are using the Manager or Assigned to option, you can allocate a colour to each staff member.
Under Settings => Users, click on the staff member's name.
Choose the colour you would like, and click on Save User Details.
Any job which is allocated/managed to/by that person will be in that chosen colour.