Slowly foraying into materials and colour rendering. Decided to start by copying this master. And doing a little doodle to see what I learned.
Start with rough grayscale, low contrast. Clipping mask --> add your hues to the shadows. Work mostly in the low saturated tones and limit it to 3-4 main hues for the shadows. Then find your saturated spots and put em in. Then find your highest values and put them in. Continue until done.
Always unify and desaturate all your colours because thats how photoshop works best. You can paint saturation and brighten up the value later.
Shapes are usually highlighted by occlusion shadow around them.
Keep your colours as simple and unified as possible, because anything you pick will blend differently with the thing underneath it. Also, if you keep your hues subtle, you won't need as much to describe variation. Bright colours require more brushstrokes.
Also, you can reuse the hues on your canvas for different materials..... Interesting.
Split it up by planes. Any form you put down, find the terminator and rough it in (with gradients if you can).
Only put down form strokes when you need to. When you can leave it as is, leave it as is. If you don't "feel" the form, then add more.
Imagine that you're actually looking at it. In real life. Think about where the rays are going, what they're bouncing off of.