It seems like Martin has this pretty well in hand, but here are some comments for what it is worth.
For most of my adult life I have been searching out original uniforms and equipment for enlisted combat soldiers from many different countries and time periods. This is the material culture of any one organization at any given time .
There are always regulations outlining what soldiers are to wear, how and when. In some cases this has been realized by some countries for short periods or at the beginning of campaigns.
However there are always several factors to take into account that will effect the appearance of the troops in the field, and in some cases even in home service. It is both informative and frustrating to read period inspection returns for military organizations. They either state that everything is in good order or that something is too small, missing or not of the current pattern. I am sure that if we ever invent a way to have a widow on the past, or a time machine there will be plenty of disappointed or even surprised historians!
1. A new uniform change or escalation in world tension that will stress industry as it steps up production. This will mean that many manufacturers will become suppliers, beyond the usual approved sources.
2.Shortages of the accepted materials or content. I could go on forever here, but lets just consider a few...
The stress of expansion of the US Army combined with blockade during the War of 1812.
The expansion of the British Army during 1914-18 and the introduction of P14 suspension to supplement the P08.
WWII Whermacht uniforms... dye lots, fabric contents, details, decorations and even modification of cut between 1933 and 1945.
3. Different dye lots even at one source. Even with the industrial might of the United States in WWII, you can lay a line of M-1943 Field Jackets and see variations in lining color, body color, button colors, placement of wearing instruction labels
4. The effects of use, campaign or home service. There can be modifications at unit level such as tailoring to fit, modifications for end use or unit modifications. This may be the case with Highland units discussed.
Other effects include the effects of soiling, washing, wear and tear and most importantly... UV light. This latter fact can and will modify color of almost anything. An example here is that almost all 'Butternut' ACW uniforms were not butternut in color when the garments were made. This is a result of UV light exposure combined with the dyes and mordants used to set the dyes.
5. Now for my guess for the color in discussion at the moment. I would vote for a gray color on the warm side rather than a cold or blue tinged color. It just makes sense for the material used, the usual budget for Army goods, and similar materials used in later periods by the British army in gray. This seems to be a lightweight and cheap garment made by an Army contractor driven to maximize their profit. For aesthetic reasons I believe miniature painters have traditionally used a gray with a strong blue cast for this period. I would guess it to be more like an early twentieth century British enlisted man's wool shirt color.
6. As far as the Guards officer private purchase tunic... I would think that this confirms my guess. The interior color is probably as close as we will see in the near future to the color as I think it might have been.
It is too bad that this primary source item surfaced after the paint masters were produced for two different companies, but my guess for the W.B. Highlanders seems to be pretty close.
What I will say is that the train has left the station for both companies and any changes now would be probably be unacceptable to collectors.
Please do not see this as a rubber stamp for what might be right or wrong with anything... just my opinion based on an educated guess.
All the Best,
Ken