Printing an Alternating Pattern to the Console
I like that you have separated the CreateGrid method out in to a reusable method. This is good.

Your logic inside the method is even quite good, despite your concerns about the duplication. There is a trick, though.... use a boolean:

    for (var row = 0; row &lt; gridSize; row++)
    {
        bool toggle = row % 2 == 0;
        for (var col = 0; col &lt; gridSize; col++)
        {
            grid[row, col] = toggle ? dark : light;
            toggle = !toggle;
        }
    }


Your print-grid method would likely benefit from some Linq, but what you have is OK.... rather, it is very readable. Linq (or using a StringBuilder) will improve performance by reducing the calls to Write.