If you can find primer in exactly the same color as the paint, then by all means go ahead and use it. However, it is unlikely that you are going to find the exact color match in paint and primer, and you do not want to make the mistake of selecting a primer that you think is the same color as the paint only to find that it is darker than the paint. It is therefore advisable to use a clear primer, but if you are concerned that the color of the item will show through the layer of paint you could always use a primer that is lighter in color than your paint.