How do I turn gray and white pixels on black lineart into a transparent gradiant?

I'm hoping my question is a simple one, as I'm strapped for time.

To put it simply, the lineart looks great from afar, and I plan on adding color to it in a layer under. But unfortunately, due to a bad tip, the lineart became filled with gray and white, covering up color and making it look horrid.

This can be fixed by turning these gray and white pixels into black and having them fade from the middle (The way most black brushes would look when used over a transparent background), but insofar I haven't found a way to do it. Even in most gradiant tutorials.

If I could get a step-by-step way to fix this, I'd be greatly appreciative.

Thank you!