Hmm... So you're trying to separate the pencil/pen lines from the paper? I don't think ArtRage has an easy solution for this (at least nothing that would work like Photoshops "color range" option). What you could do though is create a new layer over the top of the layer your sketch is on. On this new layer click the little black arrow to bring up the layers menu and then select "Blend" > "More Blend Modes" > "Multiply". Now if you paint on this new layer, the darker pencil lines will show through the color on the layer above it.
Nothing is easy to the unwilling.