Deterministic colours for Plot all

A minor cosmetic thing, but might be good for data presentation. Can the sequence of colors used when plotting multiple graphs be deterministic, i.e. the same sequence of colours are used for each Plot all call? Assuming of course that a timeseries hasn't already been plotted and assigned a color.
That would make plots like this easier to interpret, i.e. matched colors for injected current 1 & membrane potentail 1, etc.

Start by locating the Plot all entry point and the code that assigns colours to plotted time series. Verify repeated Plot all calls use the same colour sequence while preserving colours already assigned to previously plotted time series.

Written by the indexing model from the issue text.