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What colour do you think these tennis balls are?

Getty Images A close-up of three tennis balls in Madison Keys' hand.Getty Images
What colour do you see?

Are these tennis balls yellow or green?

It seems a simple question - the tennis balls are obviously yellow. But some people say they are obviously green.

And then there are the people who say they are obviously both.

So what colour are tennis balls? Fortunately, the ATP world number one has provided us with an answer.

Getty Images A close-up of Roger Federer's face during a tennis match, while he looks a tennis ball.Getty Images
Roger Federer became the oldest world number one in tennis history in February

In a video viewed almost 315,000 times at the time of writing, tennis star Roger Federer is asked what colour tennis balls are.

"They're yellow, right?" says the Swiss.

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For some people Federer's word was enough, with one person suggesting "if Roger says they're yellow, they're yellow", and another saying "I've always thought tennis balls were green, but if Roger Federer says they are yellow who am I to argue".

Despite one person's protests that if "the grass at Wimbledon is green, the balls cannot also be green", others remained unconvinced.

"Roger Federer is wrong" declared one Twitter user, while another suggested that they are a type of "fluorescent yellow" that can also be seen as green.

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So why do some people see different colours? The answer may lie in a previous optical illusion that made waves online about that white and gold dress. Or was it blue and black?

Is it white and gold? Or is it black and blue? David Sillito explains

The photo of a dress divided people on social media, with #TheDress trending worldwide in February 2015.

At the time Professor Stephen Westland, chair of colour science and technology at the University of Leeds, said the way people see colours varies hugely.

"We don't always see colour in the same way," he said. "The surprising thing is that this doesn't happen more often.

"It is possible that people could literally be seeing different colours but it's impossible to know what is in someone's head."