The reason for single HUD installs only is purely one of cost. But as the purchase and through life cost of HUDs are coming down through the introduction of new technology, we are beginning to see dual installs. This in itself opens up some new operational and certification issues that will take time to get right.
There are colour HUDs, but not in the civil market yet. Colour HUDs developed for military platforms may be subject to export restrictions (e.g. ITAR) so it's not just a simple case of putting them in a commercial aircraft.
I'm not aware of any civil HUD certified as a PFD (unlike in some mil transport aircraft). But this will change as manufacturers push to free up head down real estate for other functions. The extra integrity (10e-9) and cert requirements for HUD as PFD also pushed up the cost so why pay the extra when you've already got a certified head down PFD?
HUD as PFD has a number of advantages over head down PFD. It can display the same primary flight info (speed and altitude tapes, HSI, pitch ladder, vertical speed, ...) but has the added advantage of being conformal and much larger field of view. On the HUD you fly the flight path vector, in the HDD you fly by attitude.
HUD users would love an on/off switch on the yoke but (at least from the 737NG point of view) Boeing won't have it. If users want it, they need to pressure on the airframer to let them have it. It's not really a big deal from a design point of view.
You can push the HUD combiner forward (at least in the NG) but this isn't its recommended stow position - it's the breakaway position to comply with head impact criteria. I'm not sure but repeatedly doing this (and not the proper stow method) may put the combiner out of alignment and therefore no longer be conformal.
EVS is becoming a major driver for HUD installs, initially on bizjets (e.g. G4 and 5) but now even Boeing and Airbus seem to be interested. You can use it to get landing credit down to 100 feet on a CAT 1 or NPA - i.e. CAT 2 minimums. So there is a good business case here.