It's a nonissue. The general idea is orange for Bosch and green for GEMS. That is simply because the orange was the newest fangled thing when Bosch came out. Either will do you just fine as long as your follow the standard dos and don;ts. Flush, don;t mix and maintain.
Personally I wouldn't combine Dexcool with that copper Audi Heater core, I experienced the side effect of using that coolant on an Pontiac with lead soldered copper radiator and heater core a long time ago, it seems to attack the soldered joints producing numerous leaks. As my RR is a gems, I just carried on using conventional green anti freeze when I switched to the copper Audi Heater core (after a plastic tanked Audi heater core split open after a couple of weeks) A Bosch RR I work on has Green anti freeze in it (put in by the owner) and seems to be quite happy (no leaks)
I would stick with green and change it every two years. I don't trust that Dexcool crap. Too many 'anecdotal' stories out there of problems with that stuff. Definitely don't mix the two or it turns into sludge. I switched back to green with my aluminum radiator and I haven't had any issues in 110+ heat.
Prestone says their organic-acid ethylene glycol (orange stuff) tests have shown no problem with lead corrosion, and recommends the use of their organic-acid ethylene glycol coolant in either copper/brass or aluminum systems.
I'm not sure colour has anything to do with the type of anti-freeze. Mine has a large yellow collar around the coolant filler cap 'instructing' only Organic Acid Technology (OAT) anti-freeze to be used. OAT is kinder to aluminium engines than standard ethylene glycol based anti-freeze particularly those with a high silicate content which can 'attack' certain metals. If not OAT then a low silicate ethylene glycol based type is probably best to use. As said previously, don't mix different types and flush well before filling with your brew of choice.