GIMP :: Filling Line-drawn Shape With A Color

I have a cartoon line drawing. The black outlines are anti aliased so the edges of the lines have different grades of gray pixels on them. When I use the fill tool, and adjust the fill with the opacity and tolerance sliders, (I think that's what they're called in GIMP), I still can't get what I want. Only some of the gray pixels are colored in. Others are canceled out. Completely covered by the color, erasing some of the line.

Or some pixels remain uncolored. Just gray instead of a combination of the gray AND the fill color. What I'm looking for is an effect that looks like I placed a colored gel shaped like the cartoon character over the drawing. All the different shades of gray pixels should still be seen. Even the ones that are lighter than the fill color. With a gel, those pixels would appear darker, even though they were lighter than the fill color without the gel over them. The darkness of the color PLUS the added darkness of even the lightest gray pixels should make all the pixels appear darker. And the line would appear complete. Not partially erased.

Is it at all possible to get this effect?