[smokeonthewater]

I don't understand your question.... The gauge gets a switched positive from the ign switch OR a toggle if you prefer, the. It is grounded through the sending unit which is then grounded to bat neg.
Then the light for the gauge gets wired.... Often but not always the light has a switched positive and a constant ground

That wire you have labeled as "to switch (I thought it was black)" is not FOR the fuel gauge... It it most either taking power from that circuit (which spans the lighti circuits of ALL of your gauges) and using it elsewhere OR supplying power to turn on the lights.... You can remove it and verify that the gauges still light up.

On the gauge in your picture the black and red wires at the bottom almost definately are routed to the sending unit... Red is fuel level and black grounds to one o the screws mounting the sender.

A yellow wire at the tank does not belong there... The only other wire that should be there would be a green bonding wire.... Would connect to fill port, tank, and engine block along with any other major metal component or thru hull hardware.