This is the Tears of Artamon series by Sarah Ash.
The young painter is Gavril Andar. There is a nice summary of the first book on Sarah Ash's web site:
All Gavril Andar has ever known of life is the sunny clime of his southern home, his beautiful mother, and his love of painting. Until his peace is shattered – and his destiny decreed – by the arrival of a group of fierce clan warriors from the north. The father he has never known is dead. The man who ruled the wintry kingdom of Azhkendir, a man in whose veins ran the burning blood of the Drakhoul, has been murdered by his enemies.
Blood. The liquid that will seal Gavril’s fate. For becoming Drakhaon means not only ascending to the throne of Azhkendir, but also changing – changing, in subtle ways at first, into a being of extraordinary power and might. Becoming a dragon-warrior. One that must be replenished with the blood of innocents in order to survive.
Kidnapped by the warriors, Gavril is incarcerated in Kastel Drakhaon, with no means of escape from the isolated, ice-bound kingdom. Expected to avenge his father’s death, and carefully watched by neighbouring rulers waiting for their chance to move against him, the untested Gavril must fight to retain his human heart and soul in the face of impending war – and the dark instincts that threaten to overpower him…