I would use a medium to dark grey and use it for everything with a blanket stitch. The color of the propeller or the scoop on the backhoe. I recently did this on a quilt and it was darling - gave a little bit of a vintage look. Normally I use matching thread but it was a relief to use one color and not have to change all the time. Those blocks are so, so cute. You did a beautiful job of choosing colors and fusing. Please post a pic of the quilt when you're done.