I'm going to leave this here because it is new information on the topic.
Published on May 28, 2015
A little rusty on your color theory? Material design makes color easy. Find out more about color and material design at www.google.com/design.
The primary colors, (the 500) are colors that Google chose as the "main color" (or colors) that you base your design on. You start with the primary 500 then scale from light to dark; these colors are applied to different elements in the UI.
500s describe the dominant theme in the product (and are great for toolbars)
From there scale up to 700s for status bars
or down to a 300 for secondary information
The A colors are accent colors - brighter and more saturated encouraging interaction. These are perfect for highlighting primary action buttons, standard buttons, switches, and sliders.
What is interesting is that the information in this video seems to contradict with, or at least inconsistently expound upon, the information Google provides on their palette page mentioned above(http://www.google.com/design/spec/style/color.html#color-color-palette).
On the palette page they say "Google suggests using the 500 colors as the primary colors in your app and the other colors as accent colors"
In the video they say "The A colors are accent colors..."
Granted, subtle difference, but if Google is going to develop a style guide they should be consistent with the use of the word "accent."