Is there a way to change Voice Colors?
No, there isn’t. Because Dorico doesn’t have numbered voices, it produces the colours automatically by traversing the colour wheel by a particular interval for each new voice. We would really only be able to provide control for the starting point on the colour wheel (and perhaps the interval) to allow you to influence the colours chosen, but it wouldn’t be as simple as “voice 1 should be coloured like this, voice 2 should be coloured like this” because that’s not how Dorico thinks about voices.
Selecting colors would be a real boon, as some of the early options (bright green, cyan, etc) make notes nearly unreadable.
About 9% of all men have a red–green color blindness. For them it would be a great help.
About random color voices.
You have here two almost similar green on third staff.
I am not of the opinion that the Sibelius way is the only way or the best way. Indeed I found some of the colours chosen in that program to be quite difficult to see. But when I look at a green voice in Sibelius, I recognise it immediately as voice 2, and this I find extremely valuable.
Another vote for a mechanism to allow one to change voice colors. If you need a suggestion about how to do it, here’s one possibility (doesn’t require voices to have associated numbers):
Select any note in a given voice, right click it, select a contextual menu item that brings up a color wheel, Apply and Cancel buttons.
There it is. That’s a great on-the-fly solution. Another approach would be to include voice colors in Write/Notation Options, and be able to save your palette as the default. Preferences would be OK too.