Update colours

* Remove legacy `tone-` palette values (update to latest pasteup-palette)
* Update yellow/orange in comment-related SVGs to match latest palette value
* Remove `colour()` mixin (stop abstracting the abstractions!)
* Saner colour variables (reduce amount to those actually used/needed)
Will do… just that it is used in only a few places at the moment, and seems to add another layer of abstraction upon the layers we have already. Oh, how I remember those halcyon days when red was #f00 and blue was #00f ;-)