All metal halide lamps, when new, exhibit colour instability of this type. The effect becomes more and more noticeable with the extremes of arctube size, the smallest and the largest showing the greatest fluctuation.
It is simply due to the chemistry in the discharge tube changing during the first few hours of operation, when the halides begin to react with the formerly completely pure tungsten electrodes and quartz wall, which in your lamp is leading to the formation of the new compound scandium silicate. Additionally the halides migrate from their initial position in the arc tube to the coldest spot and this causes a more gradual change.
It is for this reason that all specified lamp data relates to 100-hour measurements. Before that time, the lamp has not stabilised and it cannot be measured.
The same is true also of the otherwise exceptionally stable ceramic metal halide lamps. These take much longer to stabilise, but typically by 150 hours all types will have reached equilibrium.
Best regards,
James.
