From Renta!

Shogo became interested in painting at an early age thanks to his father, who owns an art gallery. Born into a family of artists, his older brother is a prodigy, and Shogo is often referred to as the "prodigy's younger brother." As a result, his love for painting begins to fade. Thinking a change of scenery would be for the best, Shogo's brother contacts a friend living on a small island and arranges for Shogo to move. There, he meets Ryosei, a boy with "hands blessed by god." Shocked by the overwhelming talent gap between them, Shogo makes a move, and Ryosei... This is a complicated yet pure romance brought to life by a paintbrush under the stars.

From xiaojiangworld:

Ryousei and Shougo are involved in a continuous and ambiguous sexual relationship. By claiming Ryousei to be his, Shougo thought that it would help to change his own feelings of helplessness, regret and loneliness. Although captivated by Shougo, Ryousei himself slowly began to realize that he was just being used. An opportunity arrives for both Shougo and Ryousei to display their painting at Shougo's elder brother's art exhibition in Tokyo. For Shougo, who has a complex about his elder brother ever since childhood, his thought of "I may be able to return to Tokyo if I succeed in it" gradually turned into pressure. Beneath the starry sky, a paint brush paints a story of youthful love.