Excerpt from the Master Guide
All cards can fall under one of these three attributes. These are represented with a color and a symbol. The attributes are Shout (red), Beat (green), and Melody (blue). The symbols are pictured below (from left to right)
Shout (icon)Beat (icon)Melody (icon)

and are found in the top left of every card. They play a surprisingly important role in making your cards stronger, particularly for leveling up and for special training.

Press these for the full list

Shout (icon)Beat (icon)Melody (icon)