How can I revert my Final Cut Pro project back to its original color?

I don't know which method you used but they should all work the same.

In the below example I used the B&W effect and as such when you select the clip in the timeline the effect is shown in the Inspector.

To remove it simply deselect the tick box or drag the slider to zero or click your Delete/Backspace key.

When you create a snapshot it doesn’t change which project is open in the timeline. It often happens to people to continue editing the wrong project. I can easily imagine the wrong project being in black and white. But I fail to see how they’d BOTH become black and white…

Reverting the original to color may be extremely easy or require more work, depending on HOW you turned it into black and white in the first place. If you just added an adjustment layer (or adjustment clip in the latest version) and turned saturation down, just change that and it’s done. If you did manually to multiple clips it can be trickier. Tell us more and maybe post some screenshots

FCP automatically creates backups of all libraries. If you do File>Open Library>From Backup, it will show you the available versions. You might be able to open one that was just before the problem.

As Luis said, snapshots do not affect the original project at all. It's likely that you have a different project open. If not, please provide a screenshot to prove it.

Edit the project in color, then just export a black & white version of it. There really is no need for all this complication within FCP.

How would you export a black and white version?