Hi Carsten,

I'm sure this bug is going to be a pain to track down... :(

I'm using multiple org files and I'm clocking time and jumping between
the files for hours.  Then for no reason I can determine the colour
display of tasks breaks.

When it's broken I get black tasks alternating instead of all blue.  I
created a test.org file which shows this problem and it looks like the
following image - task '* two' is black and not blue.