Take your bckgrd 3" sqs and color 4 or z squares and on the lighter of the two mark a dotted line on the diagonal of the wrong side. Then mark a solid sew line 1/4" on each side of the dotted line. Layer one bckgrd and one color 4 or z right sides together with your marks on top and sew on the sew line. Do this to all 40 of your squares. This is easiest if you chain sew one side then the other. Clip apart. Cut on the center dotted line and open and press to the dark side. Trim these up to 2½" square. You will have 80 half square triangles. Lay these half sq triangles right side up with the color 4 or z to the top left and your bckgrd on the bottom right. Place a color 2 right side down on top of your half sq triangle. Stitch across the bottom. Do this to 80 of your color 2 sqs. Put the other 80 aside for later.
Take on of these units and with Color 4 on bottom right lay right side up, take a Color 2, 2½" sq place right side down, stitch across unit so when opened it looks as shown here.
My bckgrd color is the grey looks blue in this pic. Color 4 is the magenta.
Now take the 4½" x 2½" color 2 rectangle and sew it to the unit you made in previous sewing.