I have a 69 Z28 code 51 Dusk Blue with white stripes. The top of cowl is painted white. Is that corret?

Yes but the process changed a bit over the model run. Earlier built Z/28s with white stripes were very sloppy; the white would sometimes extend to the body seam behind the engine. They became progressively neater until they started using a mask around Feb '69. At that time the only stripe color on top of the cowl was limited to the portion that was exposed by the grilles.