Sometimes the curtains are blue because the artist likes blue.
Sometimes the curtains are blue because the artist's childhood bedroom had blue curtains and they subconsciously remind the artist of some aspect of their youth, but they've no idea that's why they wanted to draw blue curtains as they were replaced with blinds when they were pretty young, and they've forgotten about having had blue curtains, so if asked, would say they just liked blue.
And sometimes the curtains are blue because the artist wanted a blue background for space curtains, but didn't have enough time to add the stars, planets, spaceships and aliens.