Back up your claim with hard evidence. A twentieth-century Americanism, according to Eric Partridge, it originated in gambling or betting, as a challenge or to make sure that the bettor actually had enough cash to cover a bet. One writer holds that it was already common in eighteenth-century England, but his citation is not verifiable. In any event, it is unlikely that the actual hue of the money was in doubt, unless the questioner felt it might be counterfeit. More likely “color” was used figuratively for something readily identifiable.