We have seen the standard Stormtrooper colours in abundance - White, Black, and Red.
However, Phasma is the only chrome one I remember seeing.
How did she get to choose/design her own armour, and why can't anyone else?
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As per the Force Awakens Visual dictionary (indirectly sourced through the wikia, states that she'd sourced her chrome armour from Palpatine's Nabooian Yacht
Phasma wore armor coated in chromium salvaged from a Naboo yacht once owned by the Galactic Empire's Emperor Palpatine. Its polished finish helped reflect harmful radiation and was capable of deflecting weak blaster bolts, although the chromium served primarily as a symbol of past power. Phasma also believed that stormtrooper armor provided anonymity to an individual's gender, paying little heed to the antiquated belief that female stormtroopers were something new.
Star Wars Wiki - Phasma, Equiptment
This is supported in her page on the Databank
Clad in distinctive armor of salvaged chromium, Captain Phasma commands the First Order’s legions of stormtroopers
Star Wars Data Bank - Captain Phasma
While more Chromium armour possibly could be made, it seems to signify her rank amongst the commanding triumvirate alongside Kylo Ren and Hux.
Phasma served as one of the First Order’s commanding triumvirate, though she sometimes clashed with both the technocratic General Hux and the mysterious Kylo Ren.