I’ve done a little experiment, with the power of Dorico and the dropper tool in Microsoft Paint (seriously!) . My results are below, and there’s a clearer PDF attached for reference.
The top staff shows 24 voices, ordered Upstem 1, Downstem 1, Upstem 2, Downstem 2 etc. It’s an example of the palette that Dorico uses. I’ve numbered the colours, not the voices. The PDF probably does a better job of showing that the 24 colours are unique.
The 2 staff instrument is quite easy to understand. If you were to create an empty Piano instrument, Dorico would always give you a blue bar rest on the top staff (colour 1) and a red bar rest on the bottom staff (colour 2). Thus when you create additional voices on the top staff, colour 2 (red) is skipped.
The 3-staff instrument uses exactly the same colours but in a completely different order. I’ve gone on numbering in order that one can refer back to the top staff palette, but I can’t infer why Dorico would behave the way that it does here.
I generally work with voice colours switched on, and I engraved a piece for six pianos a little while ago, so I can confirm that for single stave and grand staff instruments Dorico’s consistent in its voice colouring. For three stave instruments I really haven’t any clue!
voice colours Full score 2018-07-02-155231.zip (20.7 KB)