Study the image to determine the light direction. Draw a small arrow pointing in the direction of the light to remind yourself, if necessary, and erase it once you have finished painting.
Create a new layer above the image for the gold if your painting program supports layers.
Choose a hard-edged brush and a medium-orange or yellow-brown color. Orange makes a richer, more fiery gold and brown makes a more pure yellow or green-gold. Paint the entire gold object a solid color. If your painting program has this ability, "lock" the layer when you have finished so that your paint strokes do not spill from this area.
Switch to a softer brush and gently paint the side of the gold object opposite the light source in a darker shade of brown or orange, shading as you would any other object. If there is a lot of contrast in the image, feel free to create very dark shadows. Be more delicate if there is a lot of light in the image, as there will be fewer dark shadows.
Using the same soft brush, choose a bright or pale yellow and gently paint the side of the gold object that faces the light, again, shading as you would any other object. Paint any raised areas more heavily so they appear brighter.
Blur, blend or smooth any rough areas on the gold, if necessary.
Select colors from the objects immediately surrounding the gold and very lightly paint some of the edges of the gold. You can lightly paint a little yellow on the objects, as well. This creates the appearance of shining gold reflecting other objects and the light reflecting onto other objects. Do not overuse this technique or you will obscure the look of gold.
Choose a small, hard brush and a very pale yellow or white. Add highlights to the highest points on the object or the areas directly facing the light. It is possible to overuse this technique, but adding plenty of highlights makes the gold look shinier. If you find that you have too many highlights, remove them by painting over them with a darker yellow or blurring them out.
Unlock the layer with your gold object if you locked it. Blur any highlights that seem too sharp, and blur the edge of the gold object slightly if it stands out too much from the rest of the image. This will make it match the rest of the image better.