From death star atc

Fighters have Black Health, like a TIE or an X-wing, while Capital Ships have Red Health, like an ISD or Medium Transport.

For refference, RRG page 3:

CAPITAL SHIPS
A ship that has a red health value is a capital ship.
* The Death Star is not a capital ship; it is a space station.

So every ship with red health is considered a capital ship, with the exception of the Death Star