To understand how to render you need to understand light and shadow. To understand light and shadow, you need to understand form. Studying form and how light and shadow affects it will make it much easier to add colour and shading to your images. To help you though, if you’re ever unsure about something (like where to put shadows and highlights), use reference. Reference is your friend. Before you start however, you first need to decide where the light is coming from. What I like to do is first add my base colours as a flat layer. Then to choose a shadow colour, I select the base colour and I make it a little darker and a little more saturated. I also move the hue slider out towards the closest side (so usually towards the closest red). For highlights, I do the opposite. A little brighter, less saturated (because direct light tends to wash out the base colour) and I slide the hue towards the middle (away from the red). This isn’t always the best way to select colours, but it works pretty well most of the time. Once I have my shadow colour I block out all the areas that I think will be in shadow. Then I make a darker shadow (using the same method) to add in the darker shadow areas, such as cast shadows like under the nose and chin. I would avoid using white as a highlight colour. White and black are very powerful and you need to use them sparingly. I tend to reserve white for specular highlights in the eyes or on very shiny, reflective surfaces like polished metal or glass. I wouldn’t recommend using it in the hair or on the skin. Hopefully that makes sense. But to summarize, Focus on understanding form. Even with a cartoonish figure, thinking about the objects as 3D forms will help you figure out where to place shadows and highlights. Shadows tend to be darker and more saturated than their base colour and also tend to shift outwards to the nearest edge on the hue slider. Highlights do the opposite, brighter, less saturated and hue moves slightly towards the middle. Use white and black sparingly.
