About color palette:
What are the color starting with 'A' (A100, A200)?
A stands for Accent. They are colors that accent the primary colors. Here's what Google says:
Choose your palette
Limit your choice of colors by choosing three color hues in the primary and one accent color in the secondary palette. The accent color may or may not need fall back options.
The vibrant accent colors are used for your primary action buttons as well as components such as switchers or sliders. Left aligned section icons or section titles can also take on the accent color.