The documentation is the source that can help you the most
SpriteBatch.Draw[
^]
From the error message I can see that
Square.Colors[]
is an array of strings, but you're setting the color parameter to a string, but it only accepts
Color
[
^] so you need to convert your string to
Color
[
^]
If your book tells you that you can use a string then you should go ask for your money back.