Well, if we use the same options as in Solid mode, to use Single Color / Material Color / Random Color, then users have to manually make sure they don’t make anything orange in the scene (if you use the default theme), or they need to run a command that changes all materials to not be orange. To me this solution sounds weak.
I would prefer a solution that is smarter, requires no extra steps or setup for users, and which is as clear as possible, making it always clear which is the selected and active object, and doesn’t limit using a specific set of colors.
Take an example like this. If we just keep using orange to mean selected, the third from last mesh here is the selected one:
But it doesn’t stand out at all. It’s just another color. And if you unselect an orange mesh, it looks the same as if it is selected. So, it has to be smarter than that.
We could do things like only take the hue, make unselected objects darker, and selected ones brighter, and possibly make the wire thicker:
Something like this, that is generally applicable and requires no setup and will work with any theme.
Draw some kind of box around selection:
Fill selected with a faint fill: