When the Tsars ruled Russia, we Cossacks had very strict guidelines on uniforms we wore. In fact, there was a Color Code.
My people, Terekians, wore Blue and White. Kubanians (those guys on the Ukrainian border) - Red And Black. Don Cossacks dressed in Blue and Red. Ural/Yaik host had Green and Yellow. Siberia was Green + Blue.
Today, as Russian Cossack society is still rebuilding itself from decades of Soviet rule and repression, the issue of uniforms is becoming a hot debate once again. Because we still don't have a common leadership, we also don't have strict standards on uniforms either. People, basically, wear what they like.
For example, these are Cossacks of the 'Orenburg Host' (which unites Cossack communities in Orenburg, Kurgan, and Chelyabinsk oblasts and is a part of the greater Ural/Yaik Cossack Host/Army):
But, so are these
Then there is this Don Cossack family near Voronezh
and this one in Novocherkassk
Older, more distinguished Cossacks dress in more traditional ways. They remember the Color Code, and other rules.
These are the men we learn from. God bless the elders, hope they'll be with us a long, long time.
Anyway, one has to look at culture also. Kubanian and Terekian Cossacks, of course, are more Caucasianized. We, in centures we've lived in North Caucasus, have mixed with Circassians of all sorts, Ossetians, Chechens, all different kinds of Dagestanis. That is why there one would see Cossacks in traditional Caucasian dress, like cherkeska (long, coat-like jacket of sorts, something we borrowed from, as name suggests, Cherkesy, Circassians), papakha hats, and burka (sleeveless outer 'coat' made from wolf or sheep skin. Invented by Chechens)
In this picture, the girl wears a burka, usually, men do (most women dislike them, they look 'ugly'. But, it keeps one warm at night, in the steppe