So the unrusted T-51 power armor parts are apparently a bug

[Kung Fu Man]

This'll take some explaining. All power armor sets in the base game can use a palette similar to how Combat Armor does. Color index 0.6600 is the standard one across all of them for rusted paint schemes, and is applied by the Model A/Mk. I lining options, even by the T-51. However, both the T-60 and T-51 armor sets apply a material swap called T60RawMetal and T51RawMetal accordingly right out of the box, which prevents palette changes.

 

Looking at the mod files for both sets, the T60's base lining removes this texture swap so the palette can be applied properly...however the T51's doesn't, despite applying the palette index. As a result while it's supposed to be rusted in appearance it uses the pristine look.

 

Another cute little bit related to this, the loading screen versions of the T51 power armor doesn't use this RawMetal material swap either, making it the only place to actually see the armor in its basest form.