There are two ways to convert a color photo to grayscale.
Select the photo and click the white square to convert the colors to grayscale (this is a sort of shortcut for coloring bitmaps).
The same technique can be used to apply a sepia effect or some other color effect to a color photo. To do this, select the photo, open the Color Editor and select a brown hue.
To reset the photo to its original colors, simply click the "no color" icon.
Double-click the color photo to open the photo tool. Then, enter -100 in the saturation field in the info bar.
To reset the photo to its original colors, set the saturation back to 0.