RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Coat of Arms
The Old School RuneScape Wiki also has an article on Coat of Arms.

Coats of Arms are quest items obtained from killing Paladins during the Underground Pass quest. Different paladins drop different coloured coats of arms, and both coats are used on Flames of zamorak cauldron in order to open the Gate of Iban.

The Flames of zamorak call for three coats of arms but there are only two different colours. To be safe, more than one of the same colour can be added.