Good evening, Elle!
I give way to your superior knowledge of Tube Line colours!
But, to me the clue is OK.
The setter is using his wiles to set you off at a tangent.
Olive is the green colour.
The rest of the clue sends you to the answer.
The capital starts to Circle Line are irrelevant, I think.
Just a ruse to confuse you.
And remember, there will be folk doing the puzzle who do not know of the Circle Line, or what colour it may or may not be.
"Butty" is not a word I use. I know it, though.
But, I certainly did not know a butty was a barge!