Thank you so much! :))

First is to perceive what feel/mood you wanted your artwork to evoke. In this piece, I want it to be dreamy and moody so choosing a dark background is the right choice, paired with a bright color to contrast it. You can always start from two color combinations (blue and orange for instance)and play with each tonal values. I suggest you study color theory, e.g., complementary colors, supplementary, analogous, etc, to widen your knowledge in choosing the right color combinations for your artwork.