why does mercury vapor make red flashes like metal halide?

Are you sure it was MV and not MH ? There are no halide salts in standard MV lamps, and the salts getting into the arc stream and vapourising are what makes the (usually) pink flashes. When the lamp is all warmed up and all the salts are vapourised, the mercury vapour in the arc tube gives its characteristic blueish green colour and the vapourised salts give red/pink/orange (depending on the composition). The mixture of those two main components give the white light of MH.

On the other hand, mercury vapour lamps have no halide salts so no pink flashes from that. It is possible, however, that the arc takes a bit of time to stabilise shortly after striking, during that time it will sort of flash (more like rapidly changing intensity), and if the lamp is coated, it will make the phosphor "flash" pink. This will only last for a few seconds, however.

All of our coated MV lamps have given off a pink glow just after starting, here is a 50W example. 

Are you sure it was MV and not MH ? There are no halide salts in standard MV lamps, and the salts getting into the arc stream and vapourising are what makes the (usually) pink flashes. When the lamp is all warmed up and all the salts are vapourised, the mercury vapour in the arc tube gives its characteristic blueish green colour and the vapourised salts give red/pink/orange (depending on the composition). The mixture of those two main components give the white light of MH.

On the other hand, mercury vapour lamps have no halide salts so no pink flashes from that. It is possible, however, that the arc takes a bit of time to stabilise shortly after striking, during that time it will sort of flash (more like rapidly changing intensity), and if the lamp is coated, it will make the phosphor "flash" pink. This will only last for a few seconds, however.

the lamp is a Philips 175W clear MV lamp

Well, that's gotten me stumped. I wonder if someone else might chime in with an explanation for this.

Could you post a picture of the lamp in question ? Maybe that could help clear things up...

I've seen clear lamps do this as well, but I'm not sure why either.  Even weirder I recall a DX mercury lamp that glowed purple after warming up in an gym fixture back in the 1990s.  I'm almost certain they were MV and not MH.  Is that possible?  I think it may have had gas fill impurities.  It'd also cycle for a bit after startup, but it lasted several years in this state before it was replaced.  Normally pink MH lamps fail quickly after such a dramatic color shift.

Some lamps really like to not follow the "rules" and exhibit weird operation characteristics. I've seen a 1kW MH in an outdoor flood light burn pink for close to a year before failing/being replaced. In the gym at my high school, pink burners lasted less than a week before going out. They were M59.

I’ve also seen MV do this, albeit very rarely, I’ve always thought it was tiny particles of tungsten that come off the overwinds during sputtering on start up, and burning up in the arc stream?

Another weird one is what I would call ‘cathode flare’ on a SOX lamp, where on starting a bright white xenon coloured flare travels up the entire length of one leg of the U tube!
Again this is very rare, and I’ve seen both Philips and GEC SOX do this!

Just after ignition, tere is not enough mercury evaporated to contribute, so only the way less luminous argon is radiating. On this background, the incandescent radiation of the hot electrodes may become dominant for a brief moment.
Otherwise the coated lamps tend to start weird color, as the argon radiates practically only in UV, so excites the phosphor. But because the green and yellow from the Hg arc is still missing, on a lamp with the phosphor blend designed to give balanced white at full output, these Hg components are missing so the result is orange to pinkish orange - the color just the phosphor gives off.

Otherwise if the lamp gets older, more stuff originally sputtered or evaporated from the electrodes or something that get loose, may get released into the arc stream and so radiate there. It then usually lands on something, so it forms just a brief flash.

I've just remembered that the MV high bays at where I used to work gave off a very pink glow when first switched on.

I've several 70W double ended MH lamps that gives off pinkish and orangish flashes during run-up: The Electra MH-DE 4200K RX7s 70W rebranded Venture MH-DE 70W/UVS/4K, Philips MHN-TD 70W/730, Sylvania HSI-TD 70W/D, and sometimes also my Osram HQI-TS 70W/WDL Excellence.
With most of them, these flashes are caused by sodium being vaporizing and in the Sylvania it is probably the lithium impurity, as it is a daylight rare-earths lamp.

Well, that's gotten me stumped. I wonder if someone else might chime in with an explanation for this.

Could you post a picture of the lamp in question ? Maybe that could help clear things up...

Here is the lamp.. https://www.lighting-gallery.net/gallery/displayimage.php?album=lastup&cat=13124&pos=69&pid=179207

I have a SUPER cheap Brinks 175w MV, and when it starts, it looks orange! By the way, it was so cheap it was only rated for 10,000 hours!

I have a SUPER cheap Brinks 175w MV, and when it starts, it looks orange! By the way, it was so cheap it was only rated for 10,000 hours!

LOL