The normal use of orange and orange/white wires is orange is for variable voltage to control display illumination dimming (i.e. the head unit display dims along with the other dash lights) while orange/white is a simple on/off signal of whether the dash lights are on (the head unit has only bright and dim display settings and switches based on whether the lights are on or not). Very few aftermarket head units have the variable dim capability - most just have daytime (bright) and night (dim) options.

You can connect the head unit's orange/white wire to the adapter harness orange wire and it should work fine. The HU doesn't care how much voltage is present - just whether it is or not.