I made my string blocks differently. I didn't like the seams in the white and colored strips, so I made mine without seams there.
I made the foundation the same, but then started with the triangle, cut in half with a seam allowance added. I put that on one short end of the foundation and added strips of the same colorway all the way across to the other side; then added the second half triangle, which goes on upside down from the first one. All the strips slant one way. Then I took a second foundation, put the triangle on it the other direction, added strips of the other colorway and the end triangle. You have two blocks; one slanting to the right and one slanting to the left. Sew the triangles together to make a row and rows together, matching triangles. Now you have your four triangles making a large diamond. All the seams are in the diamond piece and there are no seams in the strips.