May be this code will help:

PieChart::create([
    "dataSource"=>$this->dataStore("store"),
    "columns"=>[
        "category",
        "amount"=>[
            "style"=>function($row) {
                switch(($row["category"])
                {
                    case "surgery1":
                        return "color: #aaa";
                    case "surgery2":
                        return "color: #bbb";
                }
            }
        ]
    ]
]);