As soon as a weapon inflicts damage to an enemy airship (or even an ally) a hit marker is shown for all members on the same ship with that weapon. Size and color indicate effectiveness of the shot. Only mines have friendly fire. There are no hit markers for rams. Hit markers do not show for allied ships hits, but only for your crew.
Marker Size Edit
There are two different hit marker sizes: big and small. A big hit marker indicates an effective hit regarding the damage type matrix (value equal or above 1) against the hit component. A small hit marker, on the other hand, qualifies the hit as ineffective because a component with a part multiplier regarding the weapon's damage types below 1 was hit. Note: There are even hit markers for Flechette damage on guns and engines even though the multiplier is 0. In fact it is a very small number above zero, too small to do any visible damage.
Marker Interior Edit
Hit markers can either be full, or empty. The interior of the marker signifies who made the hit for better understanding of the hits when multiple weapons are shooting. Full markers, mean that the hit was made by you, while empty markers signify the hit was made by a different person on your ship. Note that if you leave the weapon you were firing before your shot lands a hit, it will appear as empty - as the game renders the empty markers according to if the hit was made by a weapon you are currently mounting or not.
Marker Color Edit
There are two different hit marker colors: red and white. When a shot hits the exposed hull (armor destroyed), the hit marker is red, regardless of whether the damage types of the shot are good or bad against hull (the effectiveness of the hit is shown by its size as explained above). If the hit marker is white, either a component other than the armor or hull is hit or the armor is not yet destroyed.
Size and color are independent. Therefore the size of a red hit marker always shows the effectiveness of the weapon's damage types against hull. Effectiveness of the hit (hit marker size) is also independent of the actual damage inflicted and solely based on damage types. If a shot hits within arming time only the primary damage type is used to determine the effectiveness of the hit.
A single projectile can produce multiple hit markers in its AoE radius - one for each hit component. The damage type of AoE hits is always the secondary damage type. As can be seen on the bottom of the exposed hull target picture in this page.