what is the point of having yellow line?

if the red line is making a full loop and yellow line as seen in real life works on the same tracks and vice verse whats the point of having a separate colour line there? i myself onboarded metro from poisar to reach WEH(western express highway) and i came in a freaking yellow metro while all the time i thought i was on red line and that yellow line starts at DN nagar / andheri west whatever you might prefer to say i got really frightened for a moment when i realised the interior of the metro had highlights in yellow and was really so damn confused unless i started seeing the stations outisde and matching them with the map....