I've been kindly asked to draw a proposal for the cover of http://www.cell.com/immunity (It's just a project and I'm not sure it will be accepted). So, here is my notebook for using GIMP (GNU Image Manipulation Program) to colorize a black and white drawing. Foreword:: just like everybody: I do hate gimp.
The Original Drawing
Scan the original drawing at 600dpi. Open it as a ".xcf" color image (the native file format of the Gimp).
Create a new layer with a white background under the original source (mode = multiply) and fill the shapes . The visible top layer is displayed over the layer to see the strokes.
Add a new transparent layer (mode = normal ) and create a radial gradient (transparent-black) from the nucleus to the side of the sheet. Use the eraser to highlight the pyramid-shaped proteins.
Create a new transparent layer (mode = normal ) and use the airbrushes to draw some shadows.
Merge the layers
Et voila !