Before I move onto the painting stage, let me explain the basics process of painting.
I’ll use the example of a shiny and hard material like the plate of the armor.

1. First, I paint the base color. I’ll build the color up by adding new layers on top and clipping them to this one.

2. I create a new layer called “light” and clip it to the base color below by clicking this icon.

With the [Pen] tool > [Marker] group > [Milli pen] sub tool, I paint the area that the light hits.

3. As with the “light” layer, I make another new layer for the shadows and clip it to the base layer.

4. Above this, I create a new layer for the reflected light and clip it below.

5. I then add another layer for the brightest part of the highlight.

6. On another new clipped layer, I change the blending mode to [Soft light] and use the [Airbrush] tool > [Soft] to add some soft highlight.

My layers look like this.

I imagined the light source being here.

7. I merge all the clipped layers with the base layer.

8. Using the [Watercolor] tool > [Soothing watercolor], I blend all the colors together. If necessary, I adjust the colors slightly at this stage.

9. When I want to make the material look extra glossy, I add another layer set to [Overlay] and add more highlights.

10. For areas where I want to hide the brush strokes, I use the [Filter] menu > [Blur] > [Gaussian blur] with a blur area of around 38.0.

I’ll use this method of painting in my illustrations.