A regular TileLayer with grayscale makeover.

Just use L.tileLayer.grayscale(url, options) instead of L.tileLayer(url, options).

I recommend using fadeAnimation: false option when creating the map object. Otherwise you will see a lot of flickering.

https://zverik.github.io/leaflet-grayscale/

This plugin was written by Ilya Zverev and published under WTFPL license.