I was contracted last week to re-lamp several 250w MH floods that light a building fascade downtown. They use 250w Blue and magenta colored lamps of the double-contact FC2 base type. My job was to relamp only fixtures containing blue lamps as they were reaching end of their short 6000 hour life and becoming mercury-green in color. All new lamps were purchased by owner before hand and ready to be installed when I arrived. The old lamps being replaced were made be PEC lighting in Japan and of good quality, very pure blue. The new lamps are a questionable chinese brand and when I unboxed one I had thought they accidentally sent 400w lamps as the arc tube is far too large to be 250w. In fact the arc tube is larger than most 400w lamps. Though stamped on each lamp is "MHQ-D 250w Blue". In the image you see the spent PEC lamp below (w/blacked arc tube), and the new lamp in my hand which arc tube is more than twice as large yet same power rating. The reddish indium deposits common in arc tubes of blue MH also appear to be absent in these new lamps. I relamped all 15 fixtures and drove back at night to see the performance. Color is much different than the old lamps, very purple in color with only about 6 actually pure blue. One was so pink it could almost be a magenta lamp. I understand theyre not burned in yet but they all looked purple-blue and not as bright as the old lamps either. Couldve they accidentally put 400w arc tubes in 250w lamps at the factory? Why else would the arc tube be so large?

It is not a mistake to have a bigger size arc tube in a blue lamp.  Normally when making white lamps you want the arc tube loading to be as high as possible, to have the maximum vapour pressures of the metal halides and as a result a good efficacy and colour rendering.  However with a coloured lamp, that is not always desirable.  Indium bromide is generally used for the blue colour lamps, and if the vapour pressure becomes too high, its usual intense blue line is broadened out to include other wavelengths.  Colour rendering improves of course, but the light also becomes whiter.  To get a saturated blue colour, its not desirable to have such high indium pressure, and it's therefore customary to use a larger volume arc tube.  Or to omit the usual heat reflective coatings from the arc tube ends etc.

You speak about your previous lamps becoming mercury-green in colour.  Actually what kills blue metal halide lamps is diffusion of sodium and lithium from other lamp components, into the arc tube.  The additional orange and red radiation from these impurities causes the saturation of the blue colour to decrease over time.  The problem has been solved however, by just two manufacturers : BLV and Sylvania.  Their blue lamps include a kind of "sodium getter", which is very successful in maintaining a strong, saturated blue colour light for a considerably longer time than ordinary blue lamps. 

Thanks for the info James! Ive also wondered why some blue lamps become pink/magenta early in life too. Ive got a Radium blue MH that burned saturated blue as it was supposed to for about 1000 to 1500 hours then became pink/purpleish and remained that way. Though I dont understand how the Phoenix (PEC) lamps can use such a small arc tube and white end paint and still achieve the saturated blue and good intensity througout life compared to these new lamps which I have a feeling wont stay blue for long.

Never seen blue MH lamps that turns pink/magenta/greenish mercury during life. Also I've seen indium lamps that don't contains sodium and indium.

@dor: I would expect the original blue plus the sodium (and other contaminants) orange (into greenish)/red (into magenta). And how these contaminants get there was explained by James...