If the lineart is clean, digital, and black-and-white (for best results) select your lineart layer, go the the 'Layer' menu, and select "luminance to transparency". The more white in each pixel, the more transparent it will be, so there will be no white jaggies around the black lines.
You have to set the layers to multiply for something like that. You can do that by going to the layers window(or whatever it's called). Where it say's normal(on your coloring layer). Click on it and choose Multiply.
However, when layers are mulitply'd, they will blend with other layers so you have careful how you color.