I will remember that for the future, but now I can't fix the color issue with the old file gone.

You can always make a cropped 100x100 version of the piece as the preview, it's fine!
Like this, for example:

Could you remove this from *requires revision? When I was making a thumbnail I accidentily saved over it

I will have to start over.

Oof, I thought I was safe, but I should of ran it through the color checker. I will fix.

Pretty neat iso building!
Watch out, though: your preview image is a downscaled version of your piece, on a white background and some kind of sampling, which we don't consider pixel art on Pixel Joint.
I would also watch out palette-wise, you have a few stray pixels (~154 colours).
I will put this piece in revision for these reasons, and when you change the preview image, your piece will most likely be added to the gallery!