Proper nouns as adjectives

Sometimes Proper nouns can become adjectives. Look at the following :
London taxi
Paris metro
Have you come across such words? Sometimes you will come across such proper nouns and you must recognise this and not get confused. 
Common nouns are also used and adjectives, for example:
a glass bottle 
a grass skirt
a garden gnome.
Pay attention to these and do not get confused.

"Cry Blue murder"

When we say that someone cries out blue murder when we feel that he/she is making a hue and cry about nothing significant. Nobody is quite sure why this idiom has evolved so; it is possible that the fuss was so significant that it seemed that the alarmist was in a 'Blue' funk or it may have a connection with"bloody murder". Again it may be that the murder was so cold-blooded that it chilled the observer 'blue'!!