Hi:
The short answer is, wait for it, this is soooo good: 'we don't know'. This is one that involves "it just works (or doesn't work) that way". Not "blowing you off" with this, I'm paraphrasing Deke McClelland, from Photoshop 7 Bible. Various color modes, including the bit amount options, limit the types or way of editing. This may have to do with the programming, or just the amount of info converting a particular mode leaves in the file. That would limit choices. Grayscale for instance, is limited to an 8-bit depth, and 256 levels of tone between and including black and white (or levels of gray), so there is more than a 1 bit black (line) drawing effect, but far less than the 16, 24 or 32-bit choices we have currently. So sometimes the thing to watch out for is what mode you're working in. The next thing is what layer you are on. I have often been working on a multiple layer project and noticed that the changes I was making weren't working, then realizing I was on the wrong layer. The next thing is that a particular layer is locked and that means you can't edit it. That goes for Illustrator as well as Photoshop. There is often a lock icon and a check box next to it, usually in the Layers pallet to indicate whether a layer is locked or not. The option to unlock the bottom (1st) layer, which often renames itself "background" in a newly re-opened filed, is often grayed out, or unavailable, thus permanently locking that layer. Duplicating that layer is the easiest, fastest way to unlock that layer. Gave you a short easy answer, figured you needed a fast fix and a trick to use next time.
Y'know never really noticed if saving as a .psd file automatically converts to indexed color. If it has I just converted it back.
One other tip in working with logos and type. If you are saving for the web, type generally works best in a .gif. Using a .jpg for type will create "muddy" looking type. You may already know this one.

Am I over the top, yet?

Scott