Honesty (Lunaria annua) has purple flowers but there is also the striking white flowered form which needless to say is known as White Honesty (L. annua var. alba). As I mentioned recently both self-seed around the garden though for some reason White Honesty seems to be the more vigorous of the two in this setting. That is curious because they are one and the same species but the purple is more common, in fact I hardly ever see White Honesty. 

 I'm referring to them as different forms but strictly speaking var. alba indicates that it is considered a variety i.e. below the taxonomic rank of species and subspecies but above the rank of form. Then again the white flowered Foxglove is Digitalis purpurea f. alba. Why is White Honesty a variety and White Foxglove a form??