A mistake a lot of beginners make is to draw or paint the image that they have in their mind instead of replicating what’s in front of them. But painting a realistic image means to replicate reality and not a stereotype image. Look closely and learn to transfer it to the canvas.
Here is a little example:
This is one of my first attempts in painting many, many years ago. You can see – besides all the other mistakes I have made – that I have painted the white of an eye – the sclera – white just because I ‘knew’ it is white.
But it is not white. Aside from a slight reddish tint due to blood vessels it is not white everywhere because it is a sphere. The picture below shows what I mean: since it is a spere there are areas facing the light source and areas that are on the shadow side. In the picture the left part of the sclera has a value of about 7 while the right part of it has a value of 4.