What I'm thinking the dealership did was:
-There was a TSB (Technical Service Bulletin) or a Recall on said headlamp assemblies,
-In said TSB the headlamp assembly P/N (Part number) on your vehicle got superseded by a new P/N for the assembly,
-Mazda probably has an updated headlamp assembly that prevents said fogging in the assemblies in regards to the DRL's.. hence the difference in color...
Either that or your dealership/technician is completely stupid and installed a halogen bulb in an HID housing (I'm not even sure if that's even possible??). If that was the case, did your 3 come standard with HID's from the factory? If so, then the dealership really is only authorized to use OE (Original Equipment) parts that are for your specific vehicle according to VIN, hence the standard halogen bulb instead of an HID bulb.
If that wasn't possible installing a halogen bulb in an HID system, then your bulb is probably on its way out and that they really never did replace the bulb but just the housing itself. New bulbs are going to always have a blue/white hue to it then gradually overtime will turn color to a yellow/pink color, then BAM. It's burnt out. It's possible that you either didn't notice the difference in hue before you brought it to your dealership or when you turn on your headlamps that it went from blue/white to pink/yellow instantly.
I'd approach your dealership by asking them NICELY "I've noticed my HIDs are a little discolored after you guys replaced the bulb and housing the other day. Do you have time to take a look at it and see if everything was installed correctly or that none of the parts you installed were defective?"
Being nice to a dealership service advisor will definitely go a long ways. That job is stressful and is always a breath of fresh air having somebody kind to you ask a legitimate question. Trust me, I know. I've worked as a service advisor for Acura and Subaru.
