I am using Adobe Illustrator and I have a ribbon shape that I have created using the pen tool and line segments.
I need to know how to isolate areas so I can fill them with a colour and create the illusion that the back and front of the ribbon curves through my layout.
How can I isolate parts of the shape I've created to add or change colours?
You can split your general shape into several parts to apply different fills:
1) make the desired number of copies by pasting them directly in front of the original shape: copy with Ctrl+C ---> paste to front by Ctrl+F
2) then draw a subtracting shape over one of that copies, make them both selected (but not any of the other copies below!), and use the Pathfinder panel - find a button "Minus front"
Thus you should get the desired parts of the original shape to fill them with different colors/gradients/patterns to achieve the realistic image of ribbon :)