You should try building up the colours in layers. For example on that engine, paint the metal parts silver/gold as you have done. Then go over all the metal parts with thinned down black paint. This will run off the raised areas and sit in the hollows, picking out all the shadows for you.
Then, any parts that have had too much wash, you can go over again with the gold/silver.
Once that's dry, mix some white with the silver (for the silver parts), or some white/yellow with the gold (for the gold parts), and use these to highlight any raised edges.
It'll add some depth and make it look a million times better.