Canvas extension color

I've just noticed that my Canvas Size dialog box no longer lets me select a canvas extension color, although at one time it did. It's now greyed out and the extensions are all added as transparent canvas. It's easy to work around this -- just fill the transparent areas with white (or whatever), but I'd rather add canvas and set the color at the same time. I work mostly with 16-bit RGB images, but even if I switch to 8 bit color the option is greyed out. Any ideas?

PS CS2 WinXP Pro 1GB RAM Xeon 1.5G processor.

For the canvas extension colour to work you must have a valid Background layer. If it has been renamed, promoting it to a "real" layer, the added canvas will be transparent.

Hey thanks for the quick reply. What I'm doing typically is working with complex images (lots of layers) that I then merge into a single layer and save as a new file. Apparently while doing this my single layer is not considered a valid background layer. I've tried renaming the layer to Background and even locking the transparency but that hasn't made any difference. Is there a way to make a layer a valid background layer for the purpose of coloring the canvas extension? Is there a way to tell if a layer is a valid background layer or not (besides trying to color a canvas extension)?

It will say Background in Italics. You can make a bottom layer into a Background from Layer>New>Background From Layer.

my single layer is not considered a valid background layer

Flattening the layers instead of merging them will create a valid Background Layer. Simply renaming it will not.

Thanks Ed and Chris for the education. My problem is solved, and as usual with PS there's more than one solution. I love this forum!