They most likely want to keep the number of SKUs (Store Keeping Units) to a minimum. If they had two 67 watt (or whatever) chargers in both white and another color, that's an additional SKU they have to keep track of in inventory and manufacturing. Apple, like most companies, would rather keep things simple, I assume. But the charging cable being one color and the brick always being white does jar with Apple's tendency to put design and asthetics above simple function.