If this were for a retro look game running on a modern system then there really are no rules.But the Rules that the old school pixel artists used that defined the look were simply to stay within the resolution 'budget' and the number of colours.Typically they needed to be less than 16 or less than 256 colours at any one time.There are a few tricks to accomplish this. Using two colours next to each other to 'fake' extra colours. This works better on pictures with less small detail and large gradients.