I am completely missing a simple word which means "an indication from someone that you can go ahead". Something like giving a green signal.
Go-ahead may suggest your idea of 'green light': (from TFD)
Permission to proceed:
- waiting for the go-ahead from the control tower.
"I'll wait for your approval."
1 The action of officially agreeing to something or accepting something as satisfactory:
the road plans have been given approval
they have delayed the launch to await project approvals
MORE EXAMPLE SENTENCES
Detailed schemes for each area are still being drawn up and the first wave of project approvals are expected by the end of the year.
They are also awaiting payments and approvals under various schemes.
He added that all the relevant approvals for the project are in order right down to the environmental impact assessment.
(Definition and examples from oxforddictionaries.com)
You're very close, and Josh gets even closer.
I'll wait until you give [me] the green light.
give somebody/something the green light
to give permission for someone to do something or for something to happen (often + to do sth)
They've just been given the green light to build two new supermarkets in the region. (often + to ) / The local prefect has given the green light to the dam at Serre de la Fare.
And from the CDAI (same link):
give somebody/something the green light also give the green light to somebody/something
to give permission for something to happen
She's waiting for her doctor to give her the green light to play in Saturday's game.
Usage notes: also used in the form give a green light:
The House of Representatives gave a green light to oil exploration off the East Coast.
The word you are looking for is clearance.
Official authorization for something to proceed or take place:
the aircraft hadn’t got diplomatic clearance to land in Mexico
he took off without air traffic clearance
Cue: (from TFD)
- A signal, such as a word or action, used to prompt another event in a performance, such as an actor's speech or entrance, a change in lighting, or a sound effect.
- The actor entered the stage on the director's cue.