British sources probably use the term grey because that is how the majority would have described it at the time of the CW ; grey = neutral, nondescript. Khaki as a widespread term comes in a little later and could represent a range of earthy colours- more yellowy brown in Victorian times.In British parlance Khaki now has the greenish brown character associated with Brit uniforms of WWII and post war National Service. Humbrol 159 Khaki drab is the grass colour I use for the bases but very green. In English, the colour beige is a wonderful catch all and probably is the term we would use now to describe something vague in colour and those greatcoats.
Cadogan uses an ochre wash on his waterclours to make the distinction between the overcoats of Russians and other grey overcoats so he didn't see them as grey but he starts from grey.
Vallejo's Model Color 880 Khaki Grey looks a good base colour, a kind of halfway house to develop the Russian overcoat "spectrum" from.
I agree with the Rava Italeri box art but foreground only as the background has been given a dramatic orange hue.