I've noticed with that my dimmable CFLs appear yellowish at full output and purplish at lower levels.  What causes the shift?  Is it related to the temperature of the lamp?  I don't think it's merely a perceived difference without an actual change in color.  It doesn't happen when dimming LED lamps.

I've seen the same effect with a low ballast factor ballast compared to normal ballast factor.  However, I don't see it with the energy-saving Ar/Kr lamps such as 34W T12 or 28W T8.  If anything those look slightly more yellowish despite being dimmer.

what ive been told is its just the lower mercury pressure in the lamp, now that you mention it i should try some 28W T8s on my 3 level ballast, i didn’t like how on the 30% setting the color looks dull and 3800K instead of 3500K

When dimming a triphosphors fluorescent lamp, this may yield a change in the color composition of the three phosphors, resulting in color shift.
This can also occur with halophosphors: Cool white halophosphors looks brownish when underdrivened on emergency lighting, and cool white triphosphors looks pinkish when underdrivened on emergency lighting and daylight halophosphors looks a pale grayish bluish white when used on emergency lighting.