today i was watching a 250 Watt HPS M-400-R3 fire up in a parking lot and after an hour all the lamp was doing was glowing green,much dimmer than normal,what cause this?...possibly a non-cycling HPS at the end of its useful life?

Look's like...
SON use mercury + sodium amalgam in xenon atmosphere as arc-tube fill, while the main aging mechanism is the sodium diffusion trough arc-tube wall.
Recent non-cycling SON were marketed with a feature (attained by exact mercury/sodium dose), where the sodium disappear much sooner, then the arc-tube fail, so then it effectively yield something like "pulse-start ceramic MV lamp". And when the arc-tube blacken a bit (this absorb blue at first), you get well known green glow of an old mercury...