Those dots on card icons...

The dot at the top indicates the card's highest stat:

Orange: Visual
Blue: Dance
Red: Vocal
Creamy purple: Balanced

And the dot below that indicates the card's skill type, if any:

Blue: perfect lock
Purple with slash: combo protect
Purple with chevron: combo bonus
Red: score bonus
Light green with chevron: healer
Light green with slash: life protect
Yellow: Overload

There are larger icons for individual card boxes: