I remember this came up before

If we colour speech say to green while within a block of "private line" text in a post does it stay hidden from others?

I recall mention once about the code matching colours to the background so a color use inside the private line defeats that method.

looking for confirmation of what happens now

Yes, all that falls inside the Private line remains hidden, color, format etc.

But it may not be readily apparent to the recipient that it is a private message.

Saw someone colour PM text in a game the other day ...

... yup, the two square brackets, the Private to and the list of names stay orange, but the speech that was coloured within retained its colour.  Still private and still obvious that it was a private message due to the aforementioned.

I seem to recall jase has said that tags within PMs can't be guaranteed 100%, so I suspect colour, italics and bold are OK, but I wouldn't be trying tables or quoted text.  :>

Keep in mind that if you use a Language tag is a form of private line, and nested Private lines aren't supported, when the first close square bracket comes by, it closes off the private line.

Coloring text in a spoiler invalidates the hidden nature of the spoiler however, since the spoiler makes the text the same color as the background.  Changing its color over-rides that.