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To enable the colour gradient mode in the Contouring icon we need to use contour shading and set the Contour Shade Colour Method to "Gradients". This is how it looks in the Contouring icon editor:
Having set this we need to scroll further down in the icon editor to find the gradient settings. This is what we can see by default:
To edit Contour Gradient Colour List open its helper (click on the << button):
By default the gradient list only contains one waypoint. Using the 
button we can add a new waypoint to the right of the current one and change its colour using the colour wheel: