Since the thread's going to sage anyway, here's the step-by-step on how I work with pages that make heavy use of screen tones or grey in general, like >>2890352 >>2886020 >>2878723
1 Make the white transparent. In Photoshop, you can use this technique: https://www.youtube.com/watch?v=j8tdVDqpHq8
In Paint Tool Sai, the Layer drop down menu has the "Luminance to Transparency" option, which does it all for you. Other programs, I'm sure, have their own means. >Isn't that what "Multiply" is for?
Usually, yes, but when a lot of grey is involved, it will just make your end result muddy.
2 Create a layer above your new transparent line art layer (let's call it "Mask") and set it as a clipping group / layer mask attached to the line art. This will make it so that anything you color only affects grey/black areas, and leaves the white alone.
3) Color ON the "Mask" layer. Use the darkest shades you'll be using in any particular area, as the greys become your shading for you. Be careful where you use color and where you choose to keep things black. In some cases, you may be able to stop here. The eye will fill in the blanks, as it were, and treat white areas as very light versions of whatever color is next to it.
4) If your image is looking too washed out or the original didn't provide enough material on its own, make a third layer below the other two and color as normal. Be careful, as this step is where you can lose a lot of detail from the original image, especially when it comes to texture or screen tone.