you can get gold bonuses with special temples , like the artemis line of temples which give improved missle attack so eventually when you upgrade the Artemis temple enough you will get say ..+3 attack which means you would get a gold weapon icon.
Same for other temples like Vulcan ( Scipii ) and Abdona ( SP? ) ( it's a Gaulish temple )

The highest you can get without temple bonuses , say with the Romans is silver , the temples provide the gold