It is common for the overlay planes to not be as deep as the normal planes. For example, the Indigo with Starter graphics (SGI's lowest end graphics platform) supports 8-bit deep normal planes but only provides a 2-bit deep overlay plane. Mid-range SGI graphics platforms support 4-bit overlays while the normal planes support up to 24-bit deep normal plane windows. SGI's high end Reality Engine does support an 8-bit overlay. You can expect deeper overlay planes in the future but do not be surprised by shallow overlay plane visuals.
Hewlett-Packard avoids this issue of shallow depths by have an 8-bit overlay plane.
When you write a program to use overlay planes, you should remember that the overlay planes may be substantially shallower than the default visual. In the case of a 2-bit overlay visual, keep in mind that you probably only have 3 colors to use (since one is transparent). Efficient use of color resources is important.
Also be advised that creating windows in the overlay planes generally means using visuals other than the default visual. Most X programmers always use the default visual and are not aware of the particular rules necessary to create a window in a visual other than the default. You must always specify a colormap and a border pixel color or pixmap when creating a window not using the default visual. Otherwise a BadMatch protocol error will occur. XCreateSimpleWindow will not be sufficient; you will need to use the more general XCreateWindow.