HPS lights emit high levels of red/yellow spectrum compared to most regular light bulbs. When you first look in your tent coming from a darker space, your eyes register the red/yellow light first because it lower on the spectrum.. once your eyes have a few seconds to adjust to the brighter light.. They will pick up the white and blue light and will gradually tune themselves to pick up the full spectrum of the light.. so it will appear that your lights are changing colour... but in actuality.. its just your eyes adusting to the VERY bright light... Keep in mind.. that 600W bulb puts out 10X+ more light than a standard 60W bulb.. so your eyes just arent used to seeing that drastic of a difference in light... Doesnt sound like there is a problem imo :-)
Cheers!