I have seen World War One studies that show light blue, and gray were about equal in their camouflage abilities with brown, khaki, and blue having about the same camouflage abilities.
I am not being smart, but is your view based on opinion or military studies?
The reason that many European militaries opted for versions of gray--"feldgrau" is actually rather
green and the old Austro-Hungarian "pike gray" was rather
blue--while Switzerland, Sweden, Finland, once-upon-a-time Denmark, Portugal, once-upon-a-time Romania all had gray uniforms of one or another shade of gray because the human eye is actually less capable of "seeing" certain grays... So gray occurs in nature, and of course, it resembles fog or even black powder cannon smoke... So clearly it is a better color if "camouflage" was sought.
In the American Civil War, and for the entire 19th century for the most part, the idea was to distinguish one's soldiers from those of the enemy, and to impart a military "pride" or "esprit de corps" where possible. So the "muscular bonding" of close order drill and the idea of being part of the "weapon system" of linear tactics and firing volleys, or by files or ranks or whatever was part and parcel of the selection of color for the uniform. As a corollary, the enemy's uniform became a source of derision and scorn: Hence "blue coats" or "blue bellies" versus "butternuts" or "gray backs" as pejoratives. Gray was cheaper to make. The gray uniform in U.S. practice famously arose due to shortages of blue dye to make the national uniform: blue coat with red collars, cuffs and/or facings. In the American Southeast, fighting against the British and their Native American/ Indian allies in the Creek Civil War, or "Red Stick War" a tail-coat or a shortened tail "coatee" was less practical than a roundabout jacket or simple work jacket. And gray was authorized for fatigue duty, and passed into military use. Over time, blue then became more generally the color of the regular army, while gray often became the "go to" militia color... With many salient exceptions to the "rule" of course!
The British developed khaki in the Raj in India, based on a subcontinent word for "dusty." After going to war against Boer farmers with the scarlet jacket or redcoat of Cromwell's New Model Army, it was then decided to go khaki. The khaki uniform, in turn, influenced a brown fatigue or barn coat tradition in the frontier post-Civil War army, and so khaki was selected for the uniform just ahead of the War with Spain. There were nowhere near enough modern uniforms for the expeditionary force to Cuba and later Puerto Rico and the Philippines of course, so the volunteers often wore blue. When the USMC landed at Guantanamo in 1898, led by officers with some Civil War experiences in several instances, they had a khaki campaign uniform, but there were nowhere near enough campaign hats. Many had to make due to the dark blue round hat suitable for parade ground use stateside or aboard ship... And in the heat, as you might expect, cases of heat exhaustion and sun stroke were many.
In the 19th century, many nations that had bright uniforms, or used the color blue also had a drab or gray overcoat? Mexico comes to mind, with bright blue Napoleonic uniforms but a gray blanket coat. That nations' cazadores or jaeger troops wore gray uniforms. In fact, rifle armed skirmisher troops were perhaps the forerunners of modern preoccupations with "blending in" since their clothing was often derived from that of traditional hunting ensembles, from the North American "rifleman's shirt" or frock to the German and scandinavian jaeger/hunter, or even the tweedy and green British shotgunner. Also the Russian army, green uniforms, had a gray greatcoat that the soldier was expected to use as a blanket while sleeping in the rough...