Basic Shading Technique:

The following tutorials demonstrate one technique for rendering basic plastic, metal and reflective chrome materials. It aims to break down the practice of shading the correct form of an object into more logical steps, like planning the appearance of the material on a new layer before applying it to part of your painting. Many artists learn to shade materials properly from painting still-life scenes, which is a highly-recommended way to learn because you're studying the actual material yourself, but if you don't have the time to study real-life materials that extensively, I hope the following tutorials will at least help you understand how to render their most basic forms.

This tutorial will be divided into four sections: Preparing Lighting and Materials, Rendering Basic Plastic, Rendering Basic Metal and Rendering Reflective Chrome. Obviously these materials are just a few in the huge realm of different surface properties. These three methods are only starting points towards using references and observation to produce other convincing-looking textures in your work.

Preparing Lighting and Materials:

1. To begin with I've shaded the gun with three different flat colours that suit each other. The grey-purple will be the metal, the orange the plastic and the light-blue the chrome.

2. Now I shade the background, showing the locations of three light sources. There's a main yellow light coming from high up in the top-left. From the viewer's perspective, this light is in front of the gun. There's also an ambient purple light coming from the bottom right, also in between the viewer and the gun. Finally, there's a cold blue backlight coming from the top right and far behind the gun and the viewer.

In this tutorial, the light-sources are intended to be far enough from the subject to make the light-rays from each only travel in one direction, like sunlight as opposed to a nearby lamp. This will ensure the hilights and shadows only face in the one direction as well.

Rendering Roughened Plastic:
Firstly, look at the differences in the appearance between these basic metal and plastic references, placed inside the same lighting environment. ('Basic' in this case, means that the materials show very little reflections from their environment)

The first thing you'll notice is that metal has a much brighter, larger and sharper-edged hilight. Both these materials have a rounded edge that catches the light, but the metal hilight extends along this edge a little further than the plastic and is almost completely white in colour.

The hilight on the plastic is smaller and dim enough to allow the plastic's original colour to show through. This hilight also feathers out over a wider distance than the metal hilight does.

Notice also how the shadows stretch around the sides of the steel lid further than the shadows on the plastic one. Generally, metal surfaces always show more contrast in values than plastics.

2. Try recreating that effect on a simple sphere in your image. Create a new layer and paint a circle filled with the plastic colour you've chosen.
3. First, paint the effect the warm yellow main light will have on this sphere, starting with the hilight. Open the Colour Picker and choose the colour and brightness of your main light (like a 90% Brightness, 15% Saturation orange-yellow for sunlight). Paint a point of this colour onto your sphere on the "Colour Dodge" brush setting at 80% opacity. It's important that this hilight is small. It's diameter should be around one tenth of the sphere's. Make sure this hilight is pointing towards your yellow light source, as you'll be using this sphere as a guide when you render. This will be the brightest point of the hilight. Now work a feathering effect of this colour around this hilight on the sphere so that the hilight has grown to blend in with the plastic colour.
4. Now comes the shadow from the main light. Using the wide airbrush on the 10% setting, paint a blended crescent around the opposite side of your sphere to the hilight. The colour you choose shouldn't be too dark - around 25% Brightness on the HSB scale is a good value.
5. Next, add the lights of lesser strength. The cool purple ambient light is next so as with the main light, open the Colour Picker and choose the colour and brightness of this light. This ambient light is much softer that the main but also gives out a much richer colour. For this purple ambient light, I choose a Saturation level of 45% and a Brightness level of 60% mode on the airbrush tool. Place the hilight with a brush on "colour dodge" mode at 90% opacity as before, making sure it's pointed towards the light-source. Since this light source is almost as far from the viewer as the sphere is, the hilight sits very close to the edge of the sphere and is squeezed tighter next to this edge as a result. Now feather this hilight out as before, but because this side of the sphere is entirely in shadow, the ambient light has a greater influence here and this feathering effect now covers a much wider area.
6. Finally, add the cool blue backlight's effect. Since this light is behind the sphere, it casts a blue crescent as a hilight instead of a point.

Keep working on the sphere until it looks like it belongs in the background it's painted in but don't bother with refining it to a beautifully-brushed state, since you're not going to include it in your final painting. Just make sure the colours and values are what you're going for and all the hilights correctly pointed towards their light-sources.

7. This sphere isn't just a test to see if how your plastic material will look in the scene. It can be used as a palette as well. Find the largest, flattest surface on the plastic object you're rendering and imagine which direction it's facing if it actually exists in real space. Now pick the point on the surface of the sphere that follows that same direction, colour-pick it and paint it across that flat surface. Accuracy isn't crucial at this point.
8. Keep going by finding surfaces next to one you chose and painting their colours according to the palette. I could be wasting my time writing this up, but a few weeks ago, a friend approached me asking me how to go about shading his first perspective concept-car sketches in Photoshop. After I explained this idea to him, he returned with a very impressive and accurate rendering as a first go. Maybe this will benefit you as well.
9. Since there's no true "flat" surface on this hand-grip, adjacent values are added everywhere across the object in draft-quality strokes.

10. Once you're happy with where all the draft brushstrokes are placed, refine them and blend them together to give the smooth polished surface.

11. Finally, no anti-personell weapon would be complete without a little wear and tear. Add some scratches, dent and any other plastic-like details, making sure they follow the 3D form of whatever object your applying them to. Again, you can rely on the sphere to figure out what colours the edges of the dents will be.

Preparing Linework for Colouring Rendering a Metal Surface