Chromas are a one-use item that are carried in the Miscellaneous section of your Booty panel. They come in different color types and can be used to permanently change either the primary or secondary color of any existing piece of clothing to match that of the chroma.
Chromas can only be used to dye clothing, and cannot recolor non-tradeable clothing items (such as the Savvy Hat).
Right-clicking a chroma in your inventory will bring up a small menu from which you can recolor your clothing.
The recolor is not finalized until you click 'Recolor Item', so feel free to preview a few different items and color combinations before committing!