Pale Heather Rachel Trevor Morgan Straw Hat and Stewart Parvin Outfit

For the ceremonial welcome of Chinese President Xi Jinping and Madame Peng Liyuan at Horse Guards on 20 October 2015, the Queen selected a new ensamble, comprised  of a collarless honeycomb coat with patch pockets and large lilac buttons, a streamlined heather-coloured dress, both by Stewart Parvin, and a hat in heather straw with minimal green shading on the brim, trimmed with three large straw roses and several straw looped ribbons (Daily Mail article here, scroll right down for pics of the ceremonial welcome). The Queen wore her modern yellow gold, ruby and diamond brooch:

The Queen, the Duke of Edinburgh and the Chinese Presidential couple viewing a display of Chinese items from the Royal Collection at Buckingham Palace at the start of the State visit:

On 3 December 2015, the Queen wore what definitly looks like the matching dress (compare the central folded pleat, as seen directly above, for reference) to receive ambassadors at Buckingham Palace. She wore the Amethyst Bouquet brooch. Below she is seen with Her Excellency Mrs Teresita de Jesus Vincente Sotolongo, ambassador of Cuba, as she presents her credentials:

The dress (again, notice the central box pleat) was worn again on its own for the filming of BBC 1's documentary "Elizabeth at 90 - A Family Tribute", which was aired on 21 April 2016, the Queen's 90th birthday (Daily Mail article here). Her Majesty wore the Jardine Star brooch:

The Queen repeated the whole outfit on 1 July 2016, when she arrived at the Palace of Holyroodhouse in Edinburgh at the start of her annual week-long stay. Her Majesty was presented with the Keys of the City which she returned in the traditional Ceremony of the Keys. A Guard of Honor mounted in the Forecourt of the Palace was reviewed (Daily Mail article here). Upon arrival at Edinburgh Waverley Station the Queen wore the Kent amethyst brooch; later for the Ceremony of the Keys she swapped in favour of the Royal Regiment of Scotland badge:

On 11 April 2017, the Queen repeated the outfit on a visit to Bedfordshire, during which there was time to feed the elephants at Whipsnade Zoo (Daily Mail article here). The Queen wore her daffodil spray brooch: