Basic questions are good, Julie! The only silly questions are the ones that aren't asked!
So... The way to go darker on any object, shiny or not, depends on the artist. How's that for dodging the question?
You can build darker values by continuing to layer with the same pencil, letting the white of the paper show more in areas you want to be light. But that can only get you so far.
You can use a darker pencil, and layer it in lightly, until you get to the value you want.
Or, you can use a complementary color to darken the local color. This method can give you a deepness and richness of color that the other methods don't quite get. But its also the one that takes the most practice and is the most likely to scare the bejeezus out of you.
For more info on it, read through the color wheel thread
stickied at the top of the forum. Lots of good info there.
So do some practice swatches, and experiment and find out what you're comfortable with. And practice practice practice. 'Cause what works for me may not be what works for you.