The dictionary describes plants that can be any of several different colors. But this doesn't seem to me to get to the heart of this use of the word periwinkle. I checked my infamous shirt - the one I wore that day. It was green. It not only changes color in different lights, it also has splotches of grey-green peppered through it.
Periwinkles have opposite leaves.
Another possibility, one of the several varieties of periwinkle plants has a variegated leaf. Yes, my shirt is kind of variegated with the grey-green colors peppered over the green. I could try to find a dictionary for the fabric industry. This could get very involved. I will just continue to wear periwinkles and figure that I can get by with it.