Are these tennis balls yellow or green?
网球到底是黄色的还是绿色的？
It seems a simple question - the tennis balls are obviously yellow. But some people say they are obviously green.
这似乎是一个简单的问题 - 网球显然是黄色的。但有些人说显然是绿色的。
And then there are the people who say they are obviously both.
还有人说显然是两者兼有。
An online Twitter poll that aimed to garner opinion on the hot potato found that, of 30,000 people voting, 52 percent agreed tennis balls are green, while 42 percent suggested they were yellow. A further six percent ticked 'other.'
推特上发布的一个有关网球颜色的网络调查发现，在参与调查的三万名网友中，52%认为网球是绿色的，42%认为是黄色的，还有6%选择“其他”。
So what colour are tennis balls? Fortunately, the ATP world number one has provided us with an answer.
那么网球到底是什么颜色？幸运的是，国际网球联合会排名第一的费德勒为我们提供了一个答案。
一位求知欲旺盛的球迷在一次日常签名见面会上，端着相机问了费德勒这个问题：
In the video, tennis star Roger Federer is asked what colour tennis balls are.
在视频中，网球明星罗杰•费德勒被问及网球的颜色。
"They're yellow, right?" says the Swiss.
费德勒说：“是黄色的，对吧？”
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".
对于一些人来说，费德勒的话就是标准答案，有位网友说，“如果费德勒说是黄色，就是黄色”，另一位网友说：“我一直以为网球是绿色的，但如果罗杰•费德勒说它们是黄色的，我还争辩个啥。”
有网友说，网球肯定是黄色的，因为草场是绿色，如果网球是绿色的话，我们在温网的转播画面上就看不到球。
Despite one person's protests that if "the grass at Wimbledon is green, the balls cannot also be green", others remained unconvinced.
尽管有人争辩说，如果“温布尔登的草是绿色的，网球不可能是绿色的”，但有些人还是不信服。
这似乎是有道理的，但温网的场地也不是一直都维持一个标准的绿色。
有的网友就认为费德勒说错了。
"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.
“罗杰•费德勒是错的”，一位推特用户宣称，另一位则表示它们是一种“荧光黄色”，也可以被视为绿色。
And the emoji adds further weight to the debate, clearly green in colour.
而表情符号中绿色的网球让这个争论愈发热烈。
So, who's right? Well, both parties could actually be.
那么，到底谁是对的？其实，双方都可能是对的。
The official verdict from the International Tennis Federation (ITF) says 'the ball shall have a uniform outer surface consisting of a fabric cover and shall be white or yellow in colour.'
国际网球联合会的官方文件中说：“网球应有统一的外层，表面覆盖织物，应为白色或黄色”。
However, as many have pointed out, the shade commonly used appears to be 'chartreuse', a colour between yellow and green.
但很多人指出，网球的颜色通常看起来是“黄绿色”，介于黄色和绿色之间。
这个问题也引起了专业科技公司的兴趣，研究量子点电视的nanosys公司就用专业仪器测量了一颗网球的光谱，结果显示：
不是黄，也不是绿，而是黄绿。
这个问题的答案要追本溯源。
网球曾经是白色的。
在19世纪末，网球从法国起源后，在英国贵族间成为流行运动。
而英国盛产羊毛，所以网球的颜色，就是它主要材料的颜色——羊毛白。
那时网球只是贵族间娱乐消遣的运动，鲜少涉及到竞技体育的层面。
进入到20世纪中期，网球进入公开赛年代，巡回赛和大满贯级别的赛事均涉及到了电视转播，而随着第二次世界大战后科技的迅猛发展，彩色电视开始面世。
众所周知，网球场边线的颜色是白的，白色的网球在经过或砸到边线时，彩色电视机前的观众就会瞬间从视觉上失去球的踪迹，观感非常不好。
1972年，为了转播的需要，国际网球联合会与网球制作发行商协定，给网球设计了一种专用的“视觉黄”（optic yellow ）。
经研究，这种颜色在电视屏幕上会呈现出荧光，最容易被观众识别出来。
So why do some people see different colours? The answer may lie in a previous optical illusion that made waves online about a dress. Was it white and gold? Or was it blue and black?
那么为什么有些人会看到不同的颜色？答案也许可以在之前人们对一件裙子的视错觉中找到，当时这件裙子的颜色在网上引发了争议。它是白金色的？或者，它是蓝黑色的？
The photo of a dress divided people on social media， with #TheDress trending worldwide in February 2017.
这件连衣裙的照片在社交媒体引发争论，2017年2月话题#TheDress在全球范围内成为热门。
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.
当时，利兹大学色彩科学和技术主任斯蒂芬-韦斯特兰教授说，人们看到色彩的方式差异很大。
"We don't always see colour in the same way," he said. "The surprising thing is that this doesn't happen more often.
“我们并不总是以同样的方式看到颜色，”他说，“令人惊讶的是，这中情况并不是经常发生。”
"It is possible that people could literally be seeing different colours but it's impossible to know what is in someone's head."
“人们可能真的会看到不同的颜色，但我们不可能知道它在某人脑中是什么颜色。”