I'm a bit confused about the lighting—is the source behind the characters, in front, or from the sword? That needs to be clearer.
Your colors look muddy because you're relying only on black and white for shading. That’s fine for a stylistic choice, but if you want it to feel more vibrant and less flat, try incorporating colors from the environment. For example, the girl could have some blue hues reflecting off her from the background or the boy. For the boy, adding a bit of the sword’s pink glow onto his skin or clothes would help ground them in the setting.
I also think the jackets look a bit off, mainly because the line art for the clothes doesn’t match the clean lines on the face and hands. Consistent, bold linework across the whole piece would really tie it together.
And about the hair—you might want to ease off the airbrush for highlights. Right now, it makes the lighting confusing. If the light’s coming from the sword, it should only hit the tips of the bangs. If it’s from behind, the highlights should just rim the edges of the hair. I can draw some lighting/shadow guidelines for you if that helps!