What do the light signals on the cars mean?

Solution 1:

Before the start, when the cars are chosen, the color indicates the player color.

Right before the start the light flashes according to the countdown, three times yellow, then green when the race starts.

During the race:

The light on the top changing color is an indicator of energy (health/ammo, not power):

• Green means full
• yellow means around half full
• and red means low
• flashing red means disabled

When the vehicle receives damage the light flashes according to the damage type.

A blue light indicates that the vehicle is invulnerable. This occurs at the race start and when a vehicle has recovered from being disabled.

More details can be found in the Anki Knowledge Base.