Following the advice you both gave me, i tried to find a couple of tutorials and guides to shade some basic shapes.
I tried to give a different shadow format to each angle of light to see which one was the best. What do you think?
It's tempting to place the brightest highlight pixels on the edge of an object. After all, in 2D that is the first thing that the light will hit.
But we're trying to represent a 3D shape, and that's not usually what we see in the real world.
Also, we don't normally see light come exactly from the top or the side. A common light source position is top-left.
If you place your highlight a little way into the sphere, you can give the shape more volume. The version on the top-left is an example. I normally don't like to use that many shades though, as I don't like the "banding" effect you get. And this style of using offset circles doesn't give us very realistic shade.
The top-right version uses fewer shades of grey. We see that a lot of the shape is in shadow. On this one I've used a bright and very small highlight, surrounded by darker shades. This makes the sphere look shiny, like it's made of polished metal.
(The theory: the bright spot is "specular" light, which is reflected directly into the eye, like a mirror does. The darker shades are "diffuse" light, scattered by the surface of the object. Polished metal is very reflective, like a mirror, and causes less scattering than other materials.)
For bonus points, you can add secondary (reflected) light to the dark area. This comes from the ambient environment, so should match the colour of the environment. I made it blue here, because that's a common ambient colour.
On the bottom-right, the sphere uses a bigger, but less bright highlight. The shades around it are brighter. This means less specular light and more diffuse light is reaching the eye. This makes for a less shiny, more matte surface. Matte plastic for example.
This means, to a certain extent, you can choose the basic material your shapes are made from by balancing the amount of specular, diffuse and ambient light you use.
It can be interesting to look at work by great pixel artists and think about where they're using specular, diffuse and ambient light. This scene from Owlboy for example:
https://gamingcentral.in/wp-content/uploads/2018/04/owlboy-game-review-2100x1200.jpg
