Hi Christian,
We have found a workaround for this issue.
1. In Settings -> Composing and sending -> Rich text fond color -> remember the value and reset it.
2. Edit your signature and explicitly apply the color you wish to it. Save it.
3. Set the Rich text font color value to what it was before resetting.
4 Start new message and check out the signature. It should have the default color set.
Note: If you edit your signature again you should perform the steps above in order to get it colored again.
Thanks,
Anton.