Contrary to what the wiki says, modules do not necessarily turn yellow when they hit 50% HP, as detailed in http://forum.wotlabs.net/index.php?/topic/1566-for-questions-about-crew-skills-equipment-and-modules/&do=findComment&comment=454376. What actually happens is that modules have a base HP amount and a natural regen point. So long as the module does not go below this regen point it will not appear damaged, once you pass that point it becomes yellow.
This regen point isn't the same for all modules, either. Ammo racks seem to hover around 60-70% of the normal max HP. The Type 59