You can make your tombstone appear older by painting it in a way that makes it look weathered. Stone that has spent a lot of time outside will collect dirt and mold, darkening the stone. To simulate this, we need to apply a darker color paint.
You can use dark gray or black paint. I decided to use a very dark brown, because it looks more like dirt staining. I got this color by mixing dark red paint and dark green paint. By using two colors of paint, I was able to adjust the color of the staining. That way each tombstone looks a little different.
When you apply the paint, do not cover the whole surface — weathering is always uneven. Focus on edges and carving details. The areas where water drips when it rains will be the darkest. Let the paint sit for a few minutes, then wipe it off with a paper towel. This will leave paint behind in the cracks and crevices where it would normally collect. Repeat this process one or more times to darken the details, then let the paint dry.