There are numerous ways to go about coloring manga icons, but here's one of the ways I do so most often. And this was done entirely with the touchpad on my laptop, no tablet or mouse work at all.
I know, I know - yet another manga coloring tutorial. -_-; How original, right? Probably not, but I know I often look at other people's tutorials because you never know what little tips and tricks you might pick up from them. I'm continuously learning about different ways to do things, either with the images themselves or with Photoshop. (I admit that, while I enjoy tutorials, actual program manuals? BORING~ so I often ignore and play with the programs until I learn them unless absolutely necessary.)
So, let's start with our base. I'm using this scan of Munakata Kai from Tokyo Crazy Paradise that I nabbed from the scanslations. Look at that craptastic bit of pixelated grayscale mess. Don't worry! I use crappy manga images all the time; so long as you're willing to put in the effort of cleaning things up a bit you can, too. (This is directly opposite of my stance on anime/television images for graphics making - good quality for those are a must!)
Let's crop things down a bit until you get this. Still a mess, and Asago-chan as well as some random text are still there and that's not what I'm wanting for this.
Now we need the Healing Brush Tool (I used these settings). Go over the parts you want to cover up (in this, pretty much everything other than Munakata-san). Now, we have our readied image.
Time to resize! Make any final cropping you want to and then get that sucker to 100x100.
Still not the greatest, but now we're getting somewhere. Duplicate (ctrl+j) the base at Soft Light 100%. Some images might need a lower opacity or even another duplicated Soft Light layer, but for an image like this...well. Yeah.
Before we go on, though, I recommend not merging your layers. At least, not until the end. (Even still. *shudders* I save my icon batches in .psd files with a new layer set for each icon. *is annoyingly tedious about stuff like that*)
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Seriously now, is that not much more awesome than it was? But we're not done yet. Open up a Levels adjustment layer to finish cleaning up that scan. (For this, these were my settings, but that's something that'll likely change from image to image.
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Look at that sexy bit of manflesh~ *drools* Now here's where we start rocking things.
We need a new layer, set on Multiply 100%. Label this however you want; mine is skin, though when coloring something I haven't made a specific swatch set for like I have Kurepara characters, I often use the color's hexadecimal code for reference.
Take your skin color (this was #FEF1DE) and use a hard round brush, adjusting it's size as needed to color all of the character's skin. Be sure not to cover parts other than the skin, since we're coloring over the base instead of underneath.
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Now we need a new layer (again, set on Multiply 100%). This one I label dark because we'll be using it for shadows, but first let's group it (ctrl+g) with the original skin layer. I like to do this since it lets me get a bit messy without the color going everywhere.
On your new layer, take that color and brush in some darker bits for a touch of shadow. I tend to just follow the lines and curves in addition to the image's own shading (Grassy is generally FAIL with light sources, so there's lots of BS-ing my way along).
Then to soften things up a bit, I use the Gaussian Blur filter with a 1.0 pixels radius. Just enough to help spread the shading a bit and not look too pixelated.
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I'm going simple with this image, but sometimes I do highlights or a bit of a flush to the skin with the same method.
Now we do the same for the hair and eyes, although this time instead of dark I used a layer (still with the grouping!) labeled light set on Screen 100% to break up the darkness of the image a bit. Normally, I'd do the hair and eyes separately but with this image the way it is, I won't bother since I'm using the same color.
Also, some characters have hair so dark that I put the base color on an Overlay 100% layer instead of Multiply. This gives a bit of the color without overwhelming the image with it's darkness.
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And then his GRAVE tag plate.
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There's a bit of a trick I use for choosing the color for a character's mouth. Take a pale, fleshy color and then open the Color Picker and adjust the slider until you get a reddish hue (going a bit darker, too, if need be) that's not too overwhelmingly red or pink. Often I just use the character's own skin color and then make the color adjustments for this, but if they have darker skin you'll need to account for that and go much lighter to get the right tone.
So get your color and just keep doing what you've been doing with the other color layers.
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Now some greys to shade his pants and darken his belt a bit.
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And for our final bit of coloring with this icon, his shirt.
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Here's where I generally make my decision between adding a flat color for the background or a texture. If there weren't so much white space, I'd simply go with color, but since there is I'd prefer a texture to jazz things up a bit.
What I used here was an icon texture by ownthesunshine. (Some really awesome resources can be found there, I seriously cannot pimp this comm too much, IMO.) So I just slapped it on and used a Vector Mask to erase the texture and then color in what I wanted, where I wanted.
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Now for a bit of Selective Coloring to bump our colors around a bit. These are the settings I used here.
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And one last step, adding a Hue/Saturation adjustment layer. These are my settings, which give us our final product.
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Sometimes I sharpen the image to give it a bit more oomph, but this icon doesn't need it, as far as I'm concerned.
And we've reached the end of this tutorial; if you've picked up some new things, great. If not, that's okay, too. But if you decide to give this tutorial a try, I'd love to see what you come up with!
Other icons using this technique (from this post):