You need to chip and break some of those outer tiles as well as crack a few of the inner. Brown washes for the light tiles as well as fading the darker ones will work. Also some dirt and grass clumps on the inner tiles as well/ look good so far so it will look awesome when your done.
Assuming these are ceramic tiles, some chipped paint showing the white ceramic underneath would probably be appropriate as I have seen that on irl mosaics. I\'m sure a google image search of mosaic will give you more then enough source material.
