Source:
[bgcolor=green]paragraph-1
paragraph2[/bgcolor]
result:
Notice that only the first paragraph has the specified color.
If I omit the final "[/bgcolor]", then I get an error on preview. However, there is no error for having "[/bgcolor]" at the end of the second paragraph, even though it is only applied to the first paragraph.
If bgcolor is only supposed to apply to one paragraph, then both the documentation and error message ought to reflect that. My suspicion is that it is intended to apply to the complete range embedded between "[bgcolor=green]" and "[/bgcolor]", but the current implementation is broken.
Incidently, I tried to make the subtitle:
[bgcolor] is broken
(use "peek" to see how I formatted that).
I found that wnenever I previewed, the formatting was stripped from the subtitle line as seen in the subsequent edit screen. Resubmitting for preview then resulted in an error flagged on that subtitle line. I could have kept restoring the subtitle line after each preview, but I decided that it was risky - it might break something else in the forum software.
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