In one of the British Pathé videos posted in this thread I noticed that Queen Elizabeth wore a black dress for her departure from London and changed for a white dress for her arrival in Paris.
I've stumbled across an entry about the queen's white wardrobe by Norman Hartnell for the state visit in France on a blog called "The Enchanted Manor" stating that
"in 1938, prior to a scheduled Royal Tour planned to France, the Queen’s mother the Countess of Strathmore died and the trip was postponed for three weeks. Hartnell had been commissioned to design 30 outfits for the upcoming visit but since the Royal Court Mourning period required a somber wardrobe and the colorful outfits were deemed inappropriate. This Royal Tour was extremely important to build a solid political alliance between Britain and France especially at that time due to the growing hostilities in Europe and appearances were very important in establishing a good impression. The tradition dedicated for Court Mourning allowed black clothing and after an extended period of time eventually mauve colored dresses. These colors would set the wrong tone for the visit and would also be highly unsuitable choice during the hot summer months in France. Hartnell came up with a solution to the problems when he discovered that in the past white was previously used as an acceptable color during the Court Mourning period, Queen Victoria had requested an entire white funeral was she died in 1901 and the French Queens had also worn white during their mourning periods. So, in less than three weeks new dresses were created and the Queen left London wearing somber black and arrived in Paris with an entire wardrobe of white dresses. Normally not known as a fashion icon, especially in her later years, the famous “White Wardrobe” worn by the Queen in Paris during the Royal Tour of France in 1938 caused an international fashion sensation. Hartnell had designed romantic day and evening dresses made of beautiful white fabrics such as the finest silks, chiffons, lace and tulle which were embellished with sequin and pearls. Inspired by a Winterhalter portrait of Queen Victoria, Hartnell incorporated crinoline into the dress designs and the Queen also revived a past fashion trend when she accessorized her outfits with lovely parasols to match each dress. With the great press coverage and wonderful reviews of her clothing, the Queen wanted to commemorate the success of the Paris visit by commissioning her favorite royal photographer, Cecil Beaton, to document the beautiful Hartnell dresses in a series of portraits taken in the State Rooms and gardens of Buckingham Palace."