One rule would be - must have the wood specie the hardwood flooring is made of inside the medallion. It usually happens like this but it is not enough.
The other rule would be - the transition from the surrounding hardwood floor to the medallion interior shouldn't be abrupt.
The image below shows a dark hardwood flooring with a wood medallion inlay. The essence the hardwood flooring is made of can be found inside the medallion itself. The background color is red. The separations inside the medallion are made of a pure white wood.
