Use fine silver rather than sterling; sterling is problematic. Look into using “china” paints to achieve fine lines, the only problem with them is that they are opaque. When enameling on silver, it may be necessary to use a “flux” under many colors such as reds, yellows or purples. Flux, in enameling circles, is a clear glass rather than a colored one.
Watch Ricky Franks and Sandra McEwen’s videos on Youtube, lots of good advice and techniques. If you are a jeweler, be aware that you can’t solder things once they’re enameled and if you do solder you’ll need to use eutectic, IT or hard solder as their melting points are a little higher than the melting point of the glass. Lead enamels often have more brillant and beautiful colors, but… lead.
You can put colors next to each other and even blend them but it is much like painting with sand; you’re moving grains of glass around. I do it all the time; I use the wetpacking technique. If you try wetpacking, it’s important to not let the piece dry out while you’re working on because if it does and you place wet enamels next to the dry it can make a ring. Welcome to the fun and wonderful world of enameling!