The kind of green Kekso used for his Russian troops is very light but historically it's not that wrong
When we look at a green uniform perfectly preserved like on this page :
http://www.vam.ac.uk/content/articles/m ... 720s-1917/"Coronation uniform of Emperor Alexander I, 1801, "
clic on the photo and you can see a medium green with some different shade.
When you look at :
"Coronation uniform of Alexander II, 1855, Museum "
located next to the previous one on the same page.
This military uniform turned into a more black green and is often confused by those of the Napoleonic period and Alexander I
This one :
"Uniform of the Chief Master of Ceremonies, 1797, "
on the same page is quite the kind of grass green Kekso used so it's not impossible that it was also the right colors for some others uniforms, especially cavalry where more fantasy was allowed, some officers bought their own uniform hand made by their tailor.
Anyway, as long as you enjoy your painted figure, it's always the right color !
but of course the more you have knowledge the more you want to be close to reality, everybody has his own opinion on the past as well as his own favorite shade for uniforms, it's all about taste. It's just my opinion on this subject.