[Suicide Junkie]

Combat will occur if and only if either side can see ANY of the other's ships.

Red nebulae are level 5, and the only way to engage an enemy hiding in there is to Stellar manipulate the system, and destroy the nebula.

The other nebulae / storm systems have varying levels of cloaking, from 1 to 4. It is colour dependent in the unmodded game, check systemtypes.txt for details.
(Storm X.bmp corresponds to a storm with level X cloaking)