Each weapon, for each party member, has two upgrades that can be found throughout the world you can traverse. The Chroma that is required for each upgrade can be a bit of a pain. Thankfully, the trophy is a bit sloppy in tracking (?) as it unlocked for me without having all of the upgrades (I had two left).
Chroma can be found as low drops from the light creatures, but the real location to get them would be from the Chroma Crystal formations scattered throughout the areas. They can drop anywhere from one to three and they do not have a set amount that they generally drop (it is just more often they restrict themselves to one or two). The crystals respawn every real-world hour (meaning if you exhaust all of a specific color, you can always save and exit, returning at a later time to farm more.
That being said, there are 14 weapon upgrades (Weapon Schematic 1 and 2 for each weapon type) and you shall be swimming in Chroma for some colors while hardly having any for others (like Yellow and Violet).