Bearing in mind we are talking about civil transport aircraft, if you mean brown on top and blue on bottom, some place like inside the marker on approach, then I`d guess it ain`t gone be much use.
Even in high performance combat aircraft ADI reference is a better guarantee against SD.
Here is some info on research conducted on the subject of HUD v Back up display in a static F18 sim.
http://www.lw.admin.ch/internet/luft...yhudvs.con.pdf