Something that's important to remember is that every single material around us is different and has different properties when it comes to reflecting the light. The key of shadding is interacting with the light. Independently of what kind of shadding you use, from cell type shadding, to blocks of solids and softs, etc., every material has a different behaviour when reflecting the light. The way you play with shadows and lightnings on your coloring and the color combinations can easily differenciate each different material of what you're coloring.
On this tutorial I'll show how the metallic/cromatic effect color theory works. I'll be using a soft shadding myself which requires a lot more of workd blending it, but the basics behind the effect are the same on any type of shadding.
We'll work with this picture. I choosed this one because most of what he's wearing is metallic. Is good to work with big areas first as is easier to display the effects on bigger areas.
We begin applying a layer of the 'natural' color of armor, in this case his armor would be a bright purple. Now begin to add the first layer of shadows, make sure the next layer is not much darker then the base color, the metallic effects have a wide range of tonalities of the same base color. However, While laying each coat of color make sure you never approach black nor the darkest shade of color. You'll see why later on.
As you can see on the picture above the shadding is barely noticeable. Remember to define from the beginning where your light sources are coming from. This is important because of the way metals reflect the light and the shadows. Metals have a natural reflecting effect, similar to mirrors.
Metals create a contrast between light and any other solid materials close to it that produce a reflect in relation to where the light is coming from. In this case if the metal parts are receiving light from north-east the reflections will be produced exactly on the western portions of the material.
Now, metals tend to have this natural reflection property, as stated before, however, depending of the metal in question they're rather 'opaque' by nature and the reflections tend to be less noticeable and more subtle. Once you've defined through the different 'coats' of paint/color you've laid the proper shadows of the armor, we'll proceed to work with what does make your material look like metal, the dark reflections.
Remember I said depending on where the light comes from the reflections would appear on the oposite direction?. In my picture the light's coming from north-east, and the reflections must be placed on the western-facing areas. When you're shadding remember not to use the darkest tonalities of your color spectrum, the reason is because you're going to create the reflections with the darkest tonalities of your choosen color. If you look carefully you'll notice those strokes are a dark purple. The reflections are provocked by all of Plantman's body. Metallic surfaces will reflect in the form of very contrasted dark lines any solid objects that cast any shadows on them.
Now, proceed to give the propper touches of light. Remember where is the light coming from, proceed to give the propper lightinings following the shape of the object you're shadding. One of the main properties of metals is they can contrast reflections directly next to small light sources that produce shines. To create an accurate shine, you should use a lighter tonality of the same color you're using.
You could say you've basically finished at this point. However, there's another especial property in metallic surfaces, which are present when you polish it, this increases the reflection of light that produces the shines and accents the dark reflections.
To create this effect is quite tricky and requires a lot of correct blending of the tonalities you've been using. You have to begin by darkening the reflections a bit more, use the darkest tonallity of your choosen color, as close to black you can reach without using black itself.
Proceed then to apply more lightning. Increase gradually how far the light reaches into the metal, apply coats of lighter tonalities gradually. I wouldn't recomend to use white but that is up to each person's style.
As you can see in the picture above. The area touched by light has increased. That is because when you polish a piece of metal, you're giving it a more defined mirror effect, in which the light that touches the surfice can expand easier along the object.
Another important quality of polished metals is that the light sometimes tends to 'wrap' the dark reflections on large surfaces as you can see on Plantman's legs. One of the most important things about metals is precisely the way shines and reflections interact with each other.
Now the reason why I've said not to use black and white during the shading process are because painted metals won't reflect light and shadows on white and black, but always on lighter or darker shades of their own color. Only silvery/chrome tonalities reach as far as total white and total black. But doesn't mean you can't use them if you want to.
Now, one more tricky part of handling metallic surfaces. How to set metallic yellow appart from gold. Metallic yellow is NOT gold, and sometimes is hard to make a difference between the two. Check the pictures above, the picture to the left is metallic yellow, while the picture to the right is golden. The main difference resides on the color itself. Even though they both have the same color as base, yellow leans towards reds and oranges for shadows and reflections while gold leans towards maroons and deep browns.
I choosed to keep Plantman's helmet metallic yellow and proceeded to color the rest of him. During the rest of the process you'll notice which parts of him I choosed to keep 'polished metal' and which ones are a different texture, despite the colors being the same.
Finally this is a sample of how the different textures contrast with each other rather armonically. Remember that the way you apply different shades of colors and their combinations can affect the impression of what kind of material is used. For the final, high quality version, feel free to click here
I hope this tutorial wasn't too confussing and offered some help to you. More tutorials coming soon. Feel free to suggest things you'd like to see here.