Often times in this case its not the sand that causing the fluctuation in color but rather the type of portland you are using. Especially grey portland. Batch to batch may not be truly the same color, throwing off your color. Also look at your curing regiment. How much heat did you throw at the piece when covering. Did you use the exact same method when you cured. Sometimes all these variables can be exact but the slightest thing can throw the color off just enough to not get a perfect match. Best way to combat this is too cast conjoined pieces all at once. Hope this helps.