How do I colour each individual ribbon like the finished preview for these brushes?
The very basic steps here are:
make a new square document.
make a new layer
create a rectangular selection at center of document such that the width is 100% of document, it's height is about 30%.
pick the gradient tool and choose any gradient (pick a preset for now, since this an example)
fill the selection with the gradient. make the grade top to bottom.
choose edit->transform-> warp
grab one of the bottom center gadgets and pull it up above the top gadgets.
The example you give also has lines and linear decoration layered on top of the gradient. You would do all the decoration before the warp procedure.