I wouldn't call any of your selected fabrics a true "dark". The three non-light fabrics I would classify as medium and darkish medium. Given the instructions, I suggest getting another fabric that is clearly darker than the rest, such as a very dark brown, very dark purple, navy or black. Personally, I really like quilts that include a very dark fabric; I think the contrast with the rest of the fabrics makes everything more interesting. The very dark also helps the piecing stay distinct. Sometimes without enough contrast fabrics can end up "mushing" together.