Please be aware that slate is a natural stone. Because of this, slate (as well as all natural stone) will have variance from piece to piece and from lot to lot. Different quarries, even though mined from the same part of a certain Country, will have high variation.
This is the natural beauty of the product.
Note that if you are trying to add to a slate job that was installed previously, it maybe difficult to get the same tones in any new batch of slate purchased.
Also note that slate is a sedimentary rock which is composed of compressed layers of stone. It is a natural occurrence of slate to peel or slake especially in freeze thaw areas. It is not recommended to allow water to stand on natural slate as it may cause the slate to rust due to the natural mineral components within the stone. It is highly recommended to place a good quality sealer on all slate products.