Mr van den Borne cites Hubert de Vries in his Wapens van de Nederlanden [Coats of Arms of the Low Countries], Amsterdam 1995, who confirms that the two Supporters of the Arms of the Dutch Republic were “crowned Lions” as of the beginning of the Twelve Years Truce (1609-1621) and were consequently depicted in 1659 on the coins of the Republic. After the Truce the colours of the Shield of the Dutch Republic were swapped round but De Vries gives no reasons for this change, nor any specifics on the colours of the Supporters.
A search of images of the Coat of Arms of the Dutch Republic in colour in the vast collections of the Rijksmuseum in Amsterdam reveals a map by Frederik de Wit, ca. 1665, showing the Arms in the top left corner, as shown above. The Supporters are shown in natural colours, presumably because no colours were ever specified.
The Supreme Council of the Nobility (Hoge Raad van Adel) was approached and opinions were divided, especially concerning the Supporters. The Queen thought that the golden Mecklenburg Griffin in profile was difficult to combine with the crowned Dutch lion in natural colours.
The Royal Decree of 30th September 1901, (Staatsblad no. 214a) provided for the Dutch Supporter to be a golden Lion in profile (not guardant or looking out) and with a nobleman's Crown rather than a royal one. This solution would lead to the adjustment of the Royal Coat of Arms six years later, but would also be slightly amended itself.
By 1907 there was a need felt to describe the National and Royal Coat of Arms of 1815 more accurately and to correct some inaccuracies. For instance, the original decree still only mentioned two crowned Lions as Supporters. No colour was specified and so it appears that a continuation of natural colour was presumed. Neither was there any clause to specify that the Crowns should be Royal Crowns, but this again was presumed in practice.
After long deliberation the Supreme Council decided that the Lion of Nassau on the Shield should wear a nobleman’s Crown instead of a Royal Crown and that the Dutch Supporters, like those of Mecklenburg, should henceforth be in profile and in gold, without a Crown.
After agreement was reached, two Royal Decrees were issued: one on the National Coat of Arms (10th July 1907, Staatsblad no. 181) and one on a minor modification of the Coat of Arms of Prince Henry (same date, Staatsblad no. 182) regarding the colour of the Mantling, to align it with the Royal Arms, and the dexter Supporter which lost its Crown to match again with the National/Royal Arms.
Not only have we now established the reason for the anomaly in the Dutch Supporters - they were only ever described as "Two crowned Lions" with no specifications as to colour in the first place - but also established the reason for the changes in 1907 - the establishment of Prince Henry's Arms six years earlier.
Mr van den Borne's source is his book, Wapenregister van de Nederlandse adel. Hoge Raad van Adel 1814-2014 [Armorial of the Dutch nobility], Zwolle: WBOOKS 2014, in conjunction with Coen Schimmelpenninck van der Oije, Egbert Wolleswinkel and Conrad Gietman.