Allow me to try and help with this one even though it's been only a couple of days that I've been looking at MU's code. Here is my 2 cents:

1. The problem does NOT happen with trills with segments in the file. The segments saved inside the element carry the trill color inside themselves. This explains why the trill in m.19-21 is not black.
2. However, if all the segments of a trill have their default values, the segments are not saved to the file and are recreated on open. The problem arises from the fact that the trill color from the file ( -> ) is copied into the trill's line color (see TRead::readProperties() - line 3774) but when the segments are created their color is set from the trill's color rather than the line color (in Trill::createLineSegment() - line 259). Changing the trill color in the Properties panel correctly updates both the line color of the trill and the color of the sgments (in Trill::setProperty()). For a trill, it is the line color that gets saved in the file for a trill.
   Hope this helps. :)

Hi @krasko78 - the fix in the linked commit looks good to me! It would be great if you could make a PR so we can merge this. Here's a link to the developer handbook with some instructions if needed. Thanks!

@miiizen Could you do it for me instead? I have never created a pull request to master and I believe it will take me time to find out how to do it properly, accept the necessary agreements, etc. Plus I have been having some busy days lately. It is really a small change. Thanks.

I'll do it. If you tell me your email address, I'll credit you for it too

krasyo.kiryakov@gmail.com
Maybe it is a good idea to remove my comment ("// or seg->setColor(getProperty(Pid::COLOR).valuemu::draw::Color());") ?
Thanks.

Thanks, all done

Jojo, someone else is going to approve the PR, correct?

Yes