Most drawings these days are in colour. This doesn’t really change much of your workflow or the principles behind the drawing, but it does require learning some colour theory, and how it interacts with shading your drawing.
I strongly suggest reading the Colour Theory course. If you already have lots of knowledge about colours, read on! I won’t use a lot of that colour theory here, but I do use basic concepts you need to be familiar with.
To shade using colours, a colour palette of 5 colours is usually established. In the centre of the palette, there’s the base colour, which is the average or expected colour of the object. For example, an orange has base colour orange, while an apple (most of the time) has base colour red.
To the left of this base colour, we use two shading tones. The lighter one is for soft edges or reflected shadows, while the darkest colour is used for the darkest parts. Shading tones are acquired by lowering the brightness and saturation of the base colour.
To the right of this base colour, we use two highlighting tones. The lightest should be reserved for the actual spot where light hits an object, while the other should be used around it to make the transition to base colour smooth. Highlighting tones are acquired by raising the brightness and lowering the saturation.
Additionally, sometimes the hue of shadow tones is shifted to cooler colours (such as blue or purple), and the hue of highlighting tones to warmer colours (such as red or orange).
The big takeaway from this is that shadows are actually rarely solid black or grey, but also that the colour of the shadow on an object depends on its base colour. For example, a cast shadow from a green object onto a brown floor, is a dark brown colour. The shadow is a result of light not reaching some spots of the floor, not the object’s colour somehow interacting with it.
On the other hand, highlights are a result of light, which means that if you place a green and blue object next to each other, they will have spots of reflected light on them, coloured as a mixture of green and blue.
Overall Use of Colour
Essentially, everything needs to be shaded, which means establishing such a colour palette for each object would seem ideal. Most of the time, however, using lots of different colours turns your drawing into a mess, and isn’t realistic at all.
Instead, establish a 5-colour palette for your entire drawing. These can be completely unrelated colours, as long as they look good together. All the same, however, you’ll need lighter and darker variations for lots of different objects, but the important thing is that we can now use those five colours we already have as a starting point.
Even when on a computer, achieving good colours is still best done the old way: by mixing colours.
If you want a darker version, add black to the colour. If you want a lighter version, add white to the colour. If multiple objects in your scene are interacting, mix their colours to get the right highlights and reflected lights.
Give yourself a start and end colour, and mix them to achieve a nice transition between them. Note that this can be done with any tools you like, such as colouring pencils, paint, or simulated using software on the computer.