From the research I have done and there is a lot of conflicting information about there:
True/Common Comfrey (Symphytum officinale) flowers are cream, pink, yellow, white or purplish. I have only seen the purple and pink.
Russian Comfrey Bocking #4 or #14 (Symphytum x uplandicum or Symphytum peregrinum) flowers are purple, magenta-pink, red, white or blue (that fade to pink) flowers. I have only seen the purple and white ones.
There are other varieties of comfrey with blue flowers. I have one I call "Wild Comfrey". I do not know its botanical name since I found it growing on my property.
If you have comfrey in colors other than these in a particular variety, please let me know.
Russian Comfrey is almost twice as large as True Comfrey. Russian Comfrey has sterile seeds. It is not invasive at all. True Comfrey seeds can make it somewhat invasive. Overall there are no big differences between any of the comfreys in terms of medicinal or feed value.
If you want a lot of biomass for feeding animals or making/activating compost, Russian Comfrey is better than True Comfrey. All are good.