The colon is used to indicate a separate data stream.
If you save to abc:d, then you will only be able to find a file named abc with seemingly zero size. That is because the data is actually stored in that file’s alternate data stream named d.
I had fun with LibreOffice once. A friend saved a file with a colon in the filename and didn’t understand why the file wouldn’t open when he clicked on it. Luckily LibreOffice could open it via the recent files list.