Williams Mercedes is official...

Even if doesn't have the same ring as Williams Renault or Williams Honda.

Makes you wonder what the car is gonna look like. I'm kinda tired of silver cars

I'd imagine that barring any major new massive tiltle sponsor deal which seems unlikely, then Williams will retain blue and white as these have always been Frank Williams' colours.
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The Mercedes engine deal as I understand it is a customer type partnership. It's not an "works" deal like the BMW supply was who also took over the corporate brand of the team.

Williams could sorely do with another works engine supply, the problem is that their recent track record 05 to present has been average to poor, which doesn't give manufactures confidence to put their 'flagship' efforts behind them.

It now also seems unlikely that any other manufacturer apart from Honda are returning, Toyota & BMW have said no, Ford are said to be considering it, VW Audi are unlikely to be interested because their sports car involvement is very successful and is their main focus.

So Williams are now set to be a second or third (even) line engine customer. Next year out of the four Mercedes powered teams they can expect to be no.4 in the pecking order, McLaren may be off to Honda in 15 but their strength in depth means they can't be ruled out from the championship chase.