What do you guys think of this empty space in Dorico?
Maybe Notes/Rest colors can be there for easier access?
You mean this space?
That empty space is entirely dependent on what size your screen/window is.
It strikes me that Note/Rest Colors seem like a pretty arbitrary thing to have on the top bar, particularly as they don’t apply in all five modes.
Agreed. I appreciate the openness to suggest anything with no judgement - but when it comes to deciding, my thoughts would be don’t confuse me with breaking the model too much of where things go 
Reaper (haven’t used it in a long time) used to have the ability to create your own floating palette of buttons and macros, but I guess you can buy additional tools like that now. Anyway, my suggestion would be to get one of those if you want something (or a set of somethings) quickly accessible outside of the normal menu structure.
I think that would confuse the whole concept of the five modes of Dorico.