Combine traditional with digital media
With your scanned pencil drawing open in Photoshop, create a working copy of your linework layer. Use Image> Adjustments> Levels... to remove as much visible paper grain as possible. This can degrade the quality of the pencils, and you might want to leave some grain visible, but it will affect the overall image colour.
Create a new layer, fill it with black and click Layer Mask> Hide All. Click your adjusted line work layer, Select All and Copy it. Select the black-filled layer with the mask, and click the Channels tab. Click the Layer Mask alpha channel and paste your line work. Then choose Select All and use Image> Adjustments> Invert.
Now discard the Layer Mask of the black layer. What remains will be your isolated pencil drawing. Rename this ‘ Linework’. Add a Layer Style> Color Overlay and select a sepia or dark red. Use the same colour for a subtle Outer Glow. Set this layer to Multiply and paint your colours on a new blank layer underneath it.
My father’s 70s Pentel mechanical pencil and Canson 1557 Croquis 120g paper give me the sketching feel that I’ve yet to recreate with digital tools.