How to Create a Three Color Wolf Skull in Illustrator

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Clients often ask us for mind blowing vector images with a low number of colors and no gradients. The question is how can you add depth to an illustration, give it an artistic feel, and still keep it commercial? One solution: concept, variable line thickness, contour, shadow and fill color.
In this tutorial I thought of sharing with you the process of designing an aggressive wolf skull using just white, black, and grey . I previously drew a similar one for a clothing company whose name I cannot disclose. I am using Adobe Illustrator CS3, a Wacom tablet (not compulsory), and a picture of a wolf skull.
Below is the final design we will be working towards. 
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This time I will not use a hand draw sketch, but an image of a real skull. I got mine from a museum that I've visited a while back. Note: If you try using pictures from museums or expositions please check the copyright issues with the museum's personnel before using pictures of exposed items. If you don't plan to visit a museum soon, you can access the source image of this tutorial, and use mine.
Open a new file in Illustrator (Command + N), browse on your computer to where you have the picture of the skull, select and copy it (Command + C). Go to Illustrator again and paste it (Command + V) into your Illustrator file.
Now that you have the skull image in Illustrator, you need to lower the transparency so you can trace it easier. Select the picture using the Selection Tool (V) and bring down the Opacity to 70% from the Transparency Box (Command + Shift + F10).
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Now lock the layer with the image and open a new one on top so you can start drawing the vectors.
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Before you start drawing the vector outlines you need to make sure you have the proper brushes handy. ForPLUS Members, I included a set of eight custom brushes I usually use for tracing.
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New lets start tracing! Set your Fill Color to none and Stroke Color to black.
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Bring up the Stroke panel because you'll need to change the stroke weight of the brushes in the next steps.
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Bring up the Brushes panel Window > Brushes (F5) and select the brush you will use for tracing.
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You start with the contour and then you continue with the inner shapes. For the contour you select a regular calligraphic brush (7 pt. Round) and use the Pen Tool (P) to start tracing.
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After finishing tracing the contours, now let's try to spice things up by adding stylized (jagged ) interior lines to the skull. I will start with adding them to the outer side of the drawing and continue towards the interior using Art Brush 2.
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To draw the eye socket you need to change the brush to Art Brush 12 with a stroke of 3 px and draw the outer part of it.
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Next add the jagged lines to the eye socket as well using Art Brush 4 with a stroke of 3 px.
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Next, continue adding shades (jagged lines), using different brushes and stroke weights until most of the line art is done. Feel free to experiment, and take hints from the image below.
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Now add a few lines to the interior of the drawing to give it more depth. If the skull doesn't have any, it doesn't mean you cannot draw any, use your imagination here.
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Now that you've finished drawing the lines, you have to add the grey part of the design. Before you go further; Select all (Command + A), Expand the appearance (Object > Expand Appearance), and Clean up (Command + Shift + C).
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Using the Pen tool (P) with no Stroke color and a grey fill color, start drawing towards the black outlines without overlapping them.
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Now select the gray shapes and Send to Back (Command + Shift + Left Bracket).
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Repeat Steps 15-16 until you think the amount of grey is enough to add some shading and interest to the design.
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Step 18
Now we need to add the white, so select white as a fill color. Next, select all (Command + A), bring up the paint bucket (K), and-double click on an empty area of the skull to fill the color. Finally, press the expand button.
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Below is the final image. Have fun making this!
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