multiply and overlay
The multiply and overlay processes work in similar ways. I found that overlay provides a much more efficient way of colouring if the image has grey areas as it does not colour the white at all, but otherwise a manual selection with multiply will also work.
Above I used the screen process, which works similarly to overlay but creates brighter tones.
gradient multiply + gradient overlay
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ADDING TEXTURES
Really, I think the quality of the scanned ink alone is, usually, enough of an interesting texture in its own right. Adding textures can quickly overcomplicate an image or make it look cheap and gaudy.
paper textures
The paper textures were the most successful, namely because of their subtlety. When adjusting the levels of an image post-scan the texture of the paper is often lost so it is nice to reincorporate this back. It is also interesting to see how the textures can change the hue as well as the value of a colour, though this is likely to become an obstacle.
splatter textures
It is important to note that one texture may need to be worked in a different way to suit different images (or it may not suit them at all). The left image uses overlay whilst the second uses soft light.
In the future I would like to experiment with textures more to see if they could suit my work. The idea I have in mind is to record some dry brushstrokes and apply them to solid areas of colour, much like a digital collage.