Yeah, that sounds like a ground/voltage issue. Those ballasts for HID need rock solid connections and stable voltage supplies. Any small variance in Input Voltage becomes a large variance in Output Voltage, especially when you're stepping up 13.8 Battery Voltage to as high as 20K-25K VAC to initiate the arc and the 80-90VAC to maintain the stable arc. Lower battery voltage (due to bad ground/poor power supply connection) will equate to flickering start-ups and once the capsule fires, lower light output. Lower light output will sometimes equate to a change in the Color Index (measured in Degrees Kelvin, or °K), shifting downwards (bluish-white 6,000°K, becomes more like pure white 5,000°K [natural sunlight or Daylight] by dropping to 5,500-5,000°K, a 5,000°K will drop to a slightly yellowish-white 4,500-4,000°K [the best Halogen Headlights are in the 4,250-4,500°K range, Sealed Beams 4,000°K, which is why they look so "yellowish" driving next to a 5,000°K HID car]).
I HATE those 3M Scotch-lock and knock-off T-tap connectors, I use them only as a last resort if the wire isn't long enough, or too little working room, to allow to cut the wire and use a Marine-grade, adhesive-lined heat shrink butt connector to split the output to two wires. Those T-taps can and do eventually allow a solid-strand wire to break by scoring it to make electrical contact, or cuts the outer strands of stranded wire. Not to mention they allow moisture in and thus oxidation/failure of the conductor.
I would definitely be checking all connections/connectors. Last possibility, and a long shot, is that the gas mixture of that capsule is off/mislabeled and you got a 4,000°K-ish bulb instead of a 5,000°K bulb. But highly unlikely.