The royal and usually discreet banker Coutts has belied its reputation for quiet dignity by entering the designer credit card market with a square of plastic designed to shout: I am very rich.

The new top-whack chargecard from the firm, which is now the private banking arm of Royal Bank of Scotland, hopes to sideline traditional posh liveries such as gold and platinum by using the ancient royal colour purple.

Designed by the tailor Ozwald Boateng, which will alone have cost Coutts a premium, the card hopes to draw on the origins of the colour as a Roman dyestuff that was more expensive, pound for pound, than gold. More than 10,000 murex sea snails were required to provide the purple for togas worn initially by victorious generals but later only by the emperor.

Users of the World Card face a problem unknown to Nero and company.

At first flash, the plastic might be mistaken for the humble Nectar loyalty card which got to purple first. The All Star fuel card, sure sign of a company rep rather than a jetset executive, is also a vivid mixture of purples, violets and mauves.

Coutts acknowledges it doesn't have a monopoly on the colour, but is optimistic its particular shade will change the spectrum of wealth at the upper end of credit.

Peter Littleboy, the bank's head of marketing and development, said that extras available to clients "of the purple" - in the Roman sense - would outdo rival prestige cards.

Apart from mere credit, the card includes an odd but carefully researched range of vanity offers, including a round-the-world trip by private jet and the chance to have one's biography written by a "top author".

Purple people will also have the sort of fallback described by a user of a previous Coutts premium card on the DooYoo website: his or her children were picked up from school by chauffeur-driven car, courtesy of the bank when the parent's car had a flat tyre.

Mr Littleboy said: "Coutts World is definitely a cut above, and by pushing up the standards in this market sets a new benchmark in terms of what consumers should now expect from a super-premium card."

The purple card also guarantees seats at Wimbledon through "hospitality packages", but it may be more of a marketing tool than a moneyspinner for the bank. Coutts will waive the £350 annual fee for the card for holders who spend more than £50,000 a year.