Rainbow Color Effect
This tutorial will show you a way to add a creative color effect to black and white photos. This works best with an image of a light object against a darker background. Open your image in Acorn, File ▸ Open.
Now, create a new layer on top of your image, Layer ▸ New Layer (⇧⌘N). Select the paint tool and paint some colors over the image.
Set the blending mode of this layer to Color. You can choose to leave the boundary between the colors as a hard line...
...Or apply a Gaussian Blur to the Color layer by selecting Filter ▸ Blur ▸ Gaussian Blur to smooth out the color.