Is Indigo Blue or Purple?

color study, 7x7

I took the puppy for a walk this morning, hoping to wear her out so I could have more time to do the things I'd like myself, and the sky to the west was a deep, rich blue that I would call indigo, though when I searched the web for definitions, I was surprised to find many people who believe that indigo is really nearly purple:

I have always thought that indigo was a deep blue, slightly toward the red scale, the name derived from the natural dye used on the original Levis.  More like this:

Needless to say, my color sketch didn't quite catch a true indigo, either.  Working from my fast-fading memory of this morning's sky, I veered a little toward Prussian blue and not really fully Ultramarine.  Maybe cobalt would have been a better place to start.  Still, the quick sketch managed to catch something of the mood of the tree line on the crest of the rolling field.  I think I ought to assign myself the task of gathering visual memory on my morning walks and then committing it quickly to color studies, in order to improve the feeble memory that operates now.