Orange numbers do not necessarily mean weakspots
The colour of damage numbers actually depends on both weapon sharpness and hitzone value of the part being hit, not just the latter.
For ranged weapons, which do not use sharpness, orange numbers appear when hitting hitzones of at least 45.
For melee weapons, sharpness makes it possible to see orange numbers on hitzones of at least 40. These hitzones are not shown as 3* weaknesses in the monster guide, and do not trigger Weakness Exploit. This is easy to see in the training area, on the small, white barrels.
The colour of damage numbers is also not affected by the elemental hitzone of the part being hit, except in the case of fully elemental attacks like the Charge Blade's element phials.