Based on the poetic imagery evoked by the text, here’s a color palette that reflects the themes of nature, strength, and melancholy associated with the figures of the oak and the Cossack hetman.

 <Cossack Oak Dream>
1. Earthy Green - 
                            
                            #4B8B3B
                         - A deep, natural green reminiscent of the leaves of an ancient oak, symbolizing strength and endurance.
2. Dusty Brown - 
                            
                            #7D5B3A
                         - A warm, muted brown representing the earth and the connection to the Cossack heritage, evoking feelings of nostalgia.
3. Stormy Grey - 
                            
                            #A8A8A8
                         - A soft grey that reflects the somber mood of the poem, like clouds on the horizon hinting at rain.
4. Rich Gold - 
                            
                            #D6B600
                         - A rich golden hue symbolizing the light of hope and the glory of past victories, echoing the spirit of the Cossacks.
5. Deep Charcoal - 
                            
                            #333333
                         - A dark charcoal color that adds depth and seriousness to the palette, representing the struggles faced by the characters.

This color palette captures the essence of the themes presented in the poem: the strength of the oak tree, the solemnity of the Cossack’s plight, and the depth of their connection to the land.

Would you like to explore any specific application of this palette, such as for a website, book cover, or illustration?