How to fall back to the default Color Theme in Emacs?

Try:

M-x color-theme-select

Move point (the cursor) to first line showing:

[Reset]

and press 'Enter' (return).

Alternatively, to get back to the Emacs default themes, also try:

M-x color-theme-emacs-21

or

M-x color-theme-standard

However, from your question it's not clear which theme you initially started with. For example, if you simply been using the default theme provided in the color-theme quickstart when you first (manually) installed color-theme then do this:

M-x color-theme-hober

or whatever you had specified in your .emacs file (if applicable).

That's a tricky one. What I have done before is make a color theme from your default ( there's a library out there that does it but the name escapes me (color-theme-maker) maybe?)

Then you can always switch back to it whenever you want. That's what I did, I named it default-23 or something. Good luck.