Apply a gradient, which moves from darker to lighter to darker again, horizontally and at an angle across an element.
If you want to make something look like metal, you can use this gradient technique. This will help if you're selling something luxurious, since gold and silver are expensive.
This will take some tweaking to look right, since you're trying to recreate how light interacts with metal and reflects off it. The example below moves to a bright central colour, back to dark, then immediately back again to a less bright colour, before finishing on dark, so it's a multi-step gradient.
You can use almost any colour and get a metal effect, but dark yellows and medium greys are best for gold and silver. You'll probably want to start with a darker colour on the outside edges of the gradient than you'd expect.