Question on bulbs and ballasts

[IanJ]

I have used Phoenix 250W DE bulbs in 2 fittings. One with what i think is a magnetic ballast, where the colour is a very bright white with a tint of blue. In the other fitting i think it has an electronic ballast, here the bulb is blue.
Looking at the spectural charts from Sanjay, the Phoenix bulb appears to have similar graphs with any ballast, but differ in brightness(PAR?)
What i do not quite understand is, if the visual colour to the eye appears so different, would this colour difference not be a reflection of different wavelengths?

Can any explain this?