Update: the skittle squad is finished! I got bored and came up with The Lore for them as well.
All tech-priests are proud, and those not from Mars, who strain at the leashes of their Solar cousins and resent the power of the First Forge, are even more so. Yet rare indeed is the Forge World that takes it to such lengths as that of the distant Fabius Barca.
Conquered in the long-forgotten times of the great Crusade, the prideful masters of the forge refused to bow their necks and wear Martian red, instead adopting royal purple and bronze as their colors as a calculated - and petty - act of defiance. They have maintained their own credo of techno-worship, which they maintain is vitally different from those of other forges - though even learned scholars would struggle to identify the precise doctrinal differences.
In battle, the forces of Fabius Barca display uncommon care to preserve their forces and reduce casualties, often choosing to engage from longer range than normal for Imperial forces. Similarly, the Vanguard type of Skitarii is rare among their ranks, as the inevitable effects of their rad-weaponry upon both troops and environment is viewed as an unacceptable long-term cost, and many of their vehicle designs are modified to make the pilot less…disposable, viewing accumulated experience as valuable enough to accept reduced performance. They also refuse to deploy any elements of the Legio Cybernetica - either because they regard the combat robots within their care as too precious to risk, or, more likely, that they possess none.
Regardless of their alleged restraint and mindfulness, it is less a matter of the magi of Fabius Barca actually caring for the troops under their command - more than once they have fought bitter and seemingly pointless wars with the very same forces they have preserved across a dozen lesser conflicts. It is simply a slightly longer point of view - every servitor that must be replaced with vat-grown duplicates, every Skitarii that must be forged from a young fleshling, is a cost to the calculus of war that must be accounted for.
It is rumored as well that Fabius Barca retains STC printouts and patterns for long-lost vehicles such as the Krios Battle Tank, Triaros Armored Conveyor, and Macrocarid Explorator - but these rumors have little to back them up. Perhaps, in their dusty and dismal halls, the techno-lords of the Royal Forge do possess such treasures…but if that is the case, they do not utilize them, whether from pride or stubborn refusal to turn the designs over to Mars.