I dont know how people can't understand what you're saying or the original guy who defended the idea of calling them by letters. It's not just "what you call it". They have colors and letters/numbers for a reason. Each color usually represent the street it runs down in Manhattan. (If anyone cared to notice, majority of the subway is designed to traffic everything into Manhattan first and foremost).

BDFM -6 Ave. NRQW Broadway ACE 8TH Ave. Just a few for example.

In Manhattan calling them the yellow line half way works because it's meant to show the route the trains share in that borough. But once you leave Manhattan and continue calling it the yellow line, you're asking for trouble. Tell a tourist take an uptown yellow train to forest hills and without further specific information, they'll end up in Astoria. Same with an uptown Orange train can mean the difference between a scared tourist in a rough part of the Bronx or somewhere in Hillside, Queens. Colors are a side guide mainly for appearance to tell the letters apart at huge stations like Times Square. But its best to give someone a letter.

We have too many tourists here to be vague like that. I've helped countless of them get to where they have to go. And when they used blue line, I told them the letter so someone headed to the air train to JFK on the E won't end up going past 125th Street on the A and C. God forbid the person is color blind.