This is my method of trying this.
Using a homemade silicone mold 1"x2"x3"
Fill mold fully with band color (blue here) and let cure.
On band saw, cut width wise a 1/4" strip, sand both sides.
In mold, place strip in center in a lateral way, fits right in.
Mix and pour main color and fill both sides of mold. Everything bonds together. Seams look good under magnification.