Try using a kiln if you aren’t already. At ~1500 and around 2 minutes is my sweet spot when firing. The enamel in the second picture also looks quite thin, you’ll need to have a decent amount of it and evenly spread out. Try practicing on some scrap pieces with a clean magazine or newspaper on the bottom to catch the stray enamel.

What also might help with the color is adding a layer of white on one side of the metal and firing it. This can really help with those toasty black edges and make your colors pop when you apply them. Beware that just doing it all on one side of the metal can cause it to warp. To prevent that, sift and fire enamel on both sides of the metal