CoralVue ballasts have a built in safety that prevents any dimming for about 8 minutes from startup, so the bulb and ballast are warmed up prior to the dimmer function working.
For example, if you started the bulb and put the dimmer to the lowest setting, it would still operate at normal power/amperage draw for 8 minutes, then after that time the ballast goes to the low setting. Then you can switch between the different dimmer settings.
Are you allowing this 8 minute time to pass before playing with the dimmer?
Also, my radiums become bluer the more I dim the bulb. Very noticeable contrast at lower or overdrive settings like Matt said. They get whiter in overdrive.