Several times (tho' very rarely) when working on a multi-layer image in CS3, it has suddenly "gone grayscale". I "think" that it has happened when I have hit a CTRL (or ALT) and another key on the (lower right?) keyboard . . . maybe being careless (& not looking) when hitting CTRL+0 (fit to screen). Whatever, I was not deliberately doing a specific edit.
Edit/Undo doesn't work. Nor does anything else that I can find. My work-around has been to SaveAs (.PSD) (w/o flattening) & then Open that saved file. The color returns when I do that & I can continue working.
You are toggling the displayed channels which is tied to the Alt+1/2/3 etc. or Ctrl+1/2/3 etc. keys depending on which version of AE. There is no undo because it's not a tool or editing command, just a dispaly thing.
Thanks, Mylenium. After your answer, I tested and saw that the layers shown in the pallet still are colored. Activating another layer displays that layer in color Activating the "original" layer displays it in color & everything is back to "normal".