To add to TED's answer, another reason they didn't do this is that they simply outlawed wearing purple, which was the imperial color:
The Sumptuariae Leges of ancient Rome were various laws passed to prevent inordinate expense (Latin sūmptus) in banquets and dress, such as the use of expensive Tyrian purple dye.
The modern theory of primary colors hadn't developed yet for the ancient world.
Interestingly (and surprisingly to a lot of people), it also appears that most of the ancients didn't consider blue a color, and didn't typically describe things like the sky or the seas as being blue.
Blue was a latecomer among colours used in art and decoration, as well
To the excellent answers here, I would add that cloth merchants before the industrial revolution would have aggregated the production of large numbers of suppliers and delivered products to large numbers of purchasers, including foreign purchasers. This put them in a position where they naturally ended up doing considerable amounts of factoring.
Because it is non-perishable, cloth makes good collateral.
Lenders like to have something to use for security. Land might serve this purpose, but it was monopolized by the nobility. The (middle class) "merchants" had few hard goods to put up as security for a loan, but one of them was their "stock in trade." If they didn't repay, the lender would liquidate ...