Colour distinct RNA stems/helices sequentially

The current model of RNA helix colouring shades according to helix topology numbering. This means the two helices either side of a double bulge hairpin receive the same number, and therefore the same colour. This is not always useful, since helices with 'significant' intersections (ie more than 1-2 base pairs) probably adopt a more complex structure where each side of the bulge adopts a different environment.

For these situations, it is more useful to apply a distinct shade to the contiguous helices either side of the bulge. Since the colours are mapped to helix number, we should probably employ a widely accepted numbering algorithm for this purpose.