From my experience, the colon separator for 256/rgb attributes is a fairly recent clarification to the spec. I think that's the way it was always supposed to be. But when I was initially writing the 256 color support for the Windows Console about 6 years ago, `TERM=xterm-256color` still emitted colors with just the semicolons as a separator.

The colons are more correct, but the semicolons are seemingly more broadly supported.