[OS X TeX] TexShop not syntax coloring some .tex files

[Berend Hasselman]

I have just let TexShop update itself to version 3.50 on OS X Mavericks.
Some .tex files are not syntax coloured.
The file is the root of a project.

When I switch syntax coloring off and then back on in Preferences all is ok.
When I turn off line numbering and then back on again via the menu the file also be syntax colored.

I want the file to be syntax coloured.

How can I fix this?

Berend