Grey tones back to normal

As Monika said you can't undo the change to grayscale. A suggestion for the future would be that any time that you make a major change like the one you describe to a document it is a good idea to make a copy of the file and do the change on that one. When you do so you can amend the title of the new file to designate what that version has changed. An example of that would be in a case like this: "File Name_Alt_grayscale".

Monika also mentioned Mac's Time Machine feature. Even though I've been working on a Mac for over thirty years I only recently started using Time Machine. It's amazing! Being able to get back earlier versions of a file is something that I didn't think was really necessary (I never make mistakes after all) but it has really turned out to be a very useful Macintosh feature. See https://support.apple.com/en-us/HT201250#:~:text=From%20the%20Apple%20menu%20,and%20click%20Set%20Up%20Disk..

You can't undo this when the file has been closed.

Do you have a Mac and Time Machine on? Then try that.

Is it an Illustrator Cloud file? Then try the version.