Paint can give new objects a patina that makes them look much older than they actually are. You can mix colours and painting techniques to create a weathered rusty look to almost anything, from planters and picture frames to cars. One of the benefits of using paints is that you will not damage the piece being painted. For instance, if you have a metal railing outside that you would like to look rusty, you can paint it or oxidise it. Oxidising involves introducing real rust, which in time can weaken the metal and make it unsafe. Paint is not real rust, it will just give the appearance of it.
Thoroughly clean the area to be painted. Scuff the area with a green scuff pad or some fine sandpaper. Wipe the area free of dust. This will help the paint adhere better.
Dip the sponge into the brown paint. This will be your base colour. Cover most of the area with this paint, dabbing it onto the surface to be painted. The sponge also helps give some texture to the object.
Dip a dry paintbrush into the rust coloured paint. Dab it onto a rag or paper towel to remove most, but not all, of the paint. Dab on top of the brown paint. This is a dry brushing technique, and it lets you add a little colour at a time. Since both paints are wet at this point, they will blend a bit, which is what you want.
Add some red or yellow sparingly, to highlight around the rust and brown colours. Use the dry painting technique for this as well. Do small areas at a time, stepping back to look at the overall effect. You will probably not need much in the way of red and yellow paint. If you use too much of these colours by accident, just add some more of the rust coloured paint.
If desired, spray on a matt finish clear coat to protect your work.