gdb/python: address some coding style issues in py-color.c

A few minor GNU/GDB coding style issues in py-color.c:

  - Space after '&' reference operator in one place.
  - Some excessive indentation on a couple of lines.
  - Spaces after '!' logical negation operator.
  - Using a pointer as a bool in a couple of places.

There should be no functional changes after this commit.