It should depend on the style of art you do. If you have a comics style, you can scan the line art at high res and color a low res copy of it; at the end, you can up res the color and multiply/merge it with the original black and white line art scan. The edges will look sharp, and the color -- if it's smooth and not textured -- will look fine. If your style is more textured, then increasing the resolution near the end won't work as well. You'd have to lay down your base colors in the low res phase, then up res before you get into doing the detail.