From his April 30th investiture onwards, His Majesty King Willem-Alexander will fly the royal standard. His coat of arms will be identical to that used by Queen Beatrix. Contrary to the announcement in the press release of 29 January therefore, there will be no change in the design.
The blazon is: Azure, billity or, a lion rampant of the same, armed and langued gules, crowned with a coronet of two pearls between three leaves, a sword argent with hilt or, thrusted upwards, in its right hand claw seven arrows argent with heads or, tied in a garb with ribbon or, in its left hand claw. The Royal Netherlands crown resting on the ledge of the shield, supported by two lions rampant or, armed and langued gules, placed above a ribbon azure, with the motto JE MAINTIENDRAI in gold lettering. The whole placed above a royal mantling purple trimmed or, lined ermine, tied back at the corners with gold tassled cords and issuing from a domed canopy of the same, surmounted by the Royal Netherlands crown.
The royal coat of arms, which is the same as the coat of arms of the Kingdom, has only been altered once since the foundation of the Kingdom of the Netherlands. In 1907, at the instigation of Queen Wilhelmina, the number of crowns was reduced to one, surmounting the shield. At the same time, it became possible to add the royal mantle, also surmounted by a crown. The addition of other decorative elements to the coat of arms is optional.
After her abdication Queen Beatrix will adopt the coat of arms created for her (and for her sisters) in 1938 as Princess of the Netherlands, Princess of Orange-Nassau and Princess of Lippe-Biesterfeld.
EXARANDORUM 
The mantling is blazoned purple, but drawn gules. Am I missing something?
Purple in this case doesn’t necessarily mean “purpure”. Frequently, purple is used to describe a reddish purple. In addition, there are instances when the two names are used interchangeably. For example Catholic Cardinals wear scarlet red but a man’s elevation to Cardinal is spoken of as being “raised to the Sacred Purple”.
Interesting, because today on television during the abdication here, they showed the royal flag of (now) princess Beatrix and the royal flag of (now) king Willem-Alexander and they have minor differences.
The old one being this: http://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Royal_Standard_of_the_Netherlands.PNG/768px-Royal_Standard_of_the_Netherlands.PNG
Keep in mind that the royal standard(s) or flags are different from the coat of arms. In addition, Beatrix will revert to the arms that she bore prior to being Queen. Also, the flags will represent the differences in the status of both (now) Princess Beatrix and King Willem-Alexander.
Ah, silly me, I conflated your mention of coat of arms and royal standard and took it as that the royal standard was not changing. Mea culpa.
The new royal standard will be based on that one I linked in earlier but with these changes (might have missed some aspect and not sure if I got the expressions correct in English). The new one: instead of a rosette, a bow in (of?) the ribbon of the Military William Order will be used. Furthermore the tassels of the horns will identically be hanging down.