Menu -> Project Settings -> Price Model -> Colour & Custom Status
To change the colours for existing statuses, simply click on the existing color under each status. The color on the upper row is the background colour, and the colour below is the text colour. Clicking on the color under each status allows you to select your desired new colour. Once you have selected your new colour, click save.
To add a new custom status and associate a new color coding system to the new status, simply click on Add New Custom Status, give the new status a Label and select the Background Color and Text Color that you would like to associate with this new custom status before clicking on Save.