Uniform default chain colors

[AntonOresten]

Currently, default chain color is the :jet colorscheme, with a linear range from 0 to 1.
When there are multiple chains, it is much more nicer if they're each one solid color.
Something like:

colormap = [:royalblue, :violet, :limegreen, :chocolate1]
colors = [repeat([i-1], l) / (length(colormap)-1) for (i, l) in enumerate(length.(structure))]
ribbon!(scene, structure; colors, colormap)

or the RGB values directly such that changing the colormap doesnt mess up the colors.