Tinctures
Heraldry uses a limited and specific palette of colours called tinctures. These are usually divided into metals and colours. The metals are gold and silver, often shown as yellow and white. All of the others are colours.
All colours and metals are traditionally shown clearly and without much variation so that a design can be recognised at a distance. It has been a rule for centuries, the Rule of Tincture, that one does not overlap the two metals, or any two of the colours. This is not an arbitrary, arcane rule, but a recognition that some colours do not show up well against others at a distance. This concept is still applied in modern artistic theory to this day.
The palette of heraldry is made up of clear colours easily recognised by most people. Other colours, such as orange, brown, or pink, do occur in heraldry, but they are rare, as they could cause confusion. At a distance, people could misinterpret orange, for example, as either a shade of red or yellow. In medieval battles, lives depended on recognising colours correctly, so it was a good idea to avoid uncertainty!
It is important to note that in heraldry, all artistic representations of the blazon are equally valid, so long as they accurately represent the design. This means artists have stylistic licence, as long as their choices are not likely to be misinterpreted. An artist is free to use any shade of green that he or she wishes when depicting a Vert shield. However, if the shield is shown as teal, or lime, people might misinterpret the colour as blue or yellow, which would not represent the blazon correctly.