I had a look in the regimental history (Kreuzwendedich von dem Borne, 1896).
On 10 June 1815, the 27. Infanterie-Regiment had an effective strength (without 4 officers, 153 rank and file, who were ill or detached):
1st battalion: 23 officers, 730 rank and file
(all the soldiers and a few NCOs from the former Ersatz-Bataillon Nr. 7, most NCOs from the former battalions Reiche and Hellwig)
2nd battalion: 20 officers, 527 rank and file
(about 300 men from the Reserve-Bataillon of the Elb-Infanterie-Regiment, which was formed in early 1814 from the remnants of the former Royal Westphalian 1. Infanterie-Regiment, the rest from rank and file of the former battalions Reiche and Hellwig)
Füsilier battalion: 19 officers, 746 rank and file
(9th and 10th company from the former Jäger-Bataillon von Reiche, 11th and 12th company from the former Jäger-Bataillon von Hellwig.
The Royal Westphalian 1. Infanterie-Regiment took part in the 1812 campaign (X corps, Macdonald) and then was trapped in Danzig, which was handed over to the Allies on 28 December 1813. Their uniforms had been issued mid-1812 or earlier, so they were due to be replaced mid-1814 or earlier. I presume (no positive evidence) they were issued the regulation uniform of the Elb-Infanterie-Regiment: dark blue with red facings and yellow shoulder straps.
The Ersatz-Bataillon Nr. 7 was raised in late 1813 in the provinces between Elbe and Weser. Its men were destined for the 2. Westpreußisches Infanterie-Regiment and the 7. Reserve-Infanterie-Regiment, so they should have worn dark blue uniforms with carmine facings and red shoulder straps. There is no explicit info about their uniforms.
The so called "reservist uniforms" of spring 1813 were an improvisation of that period only, and I don't know of any Prussian unit in gray uniforms during the 1815 campaign.
The regulation facings of the new 27. Infanterie-Regiment were "lichtblau" (clear blue) with red piping, and red shoulder straps.
According to the regimental history, the fusileer battalion retained its green uniforms, but added the light blue "collars" [and probably cuffs as well] in May 1815. The former Hellwig companies retained their white buttons and shako chinscales until August 1815.
The other battalions had coats of Prussian and British cut, and of "completely different blue colours" (shades of blue). No white, let alone gray uniforms are mentioned. At least the "correct facings" were added. However, as late as mid August 1815, "the last red collars" were replaced with the correct light blue ones.
When the Jäger-Bataillon von Reiche marched out of Berlin on 6 April 1813, its uniforms were not yet ready. Therfore it marched out in white jackets of British origin. These jackets (a kind of counterpart to the gray "reservist jackets") were meant to be worn under the uniform in winter, or off duty. The regimental history doesn't mention any of them in 1815, but some of them may still have been in use in the 9th and 10th companies, worn under the uniform or when off duty.