After more than 25 years of searching, neuroscientists in the UK recently announced that they've discovered a woman who has an extra type of cone cell - the receptor cells that detect colour - in her eyes.

According to estimates, that means she can see an incredible 99 million more colours than the rest of us, and the scientists think she's just one of a number of people with super-vision, which they call "tetrachromats", living amongst us.

Most humans are trichromats, which means we have three types of cone cells in our eyes.

Each type of cone cell is thought to be able to distinguish around 100 shades, so when you factor in all the possible combinations of these three cone cells combined, it means we can distinguish around 1 million different colours.

Most people who are colour blind only have two functioning types of cone cells, which is why they can only see around 10,000 shades - and almost all other mammals, including dogs and New World monkeys, are also dichromats.

But there's one doctor in northern England who has four cone cell types, taking the potential number of colours she can distinguish up to 100 million - colours most of us have never even dreamed of.

Identified only as cDa29, the scientists finally found this woman two years ago, but they've been searching for more than 25 year - and think there are more tetrachromats like her out there.

So how do you get a fourth type of cone cell?

The idea of tetrachromats was first suggested back in 1948 by Dutch scientist HL de Vries, who discovered something interesting about the eyes of colour blind people.

While colour blind men only possess two normal cone cells and one mutant cone that's less sensitive to either green or red light, he showed that the mothers and daughters of colour blind men had one mutant cone and three normal cones.

That meant they had four types of cone cells, even though only three were working normally - something that was unheard of before then.

Despite the significance of the finding, no one paid much attention to tetrachromats until the late '80s, when John Mollon from Cambridge University started searching for women who might have four functioning cone cells.

Assuming that colour blind men pass this fourth cone cell onto their daughters, Mollon estimated that around 12 percent of the female population should be tetrachromats.

But all of his tests showed that these women could only perceive the same colours as the rest of us - which meant only three of their cone cell types were working, so they weren't true tetrachromats.

Then, in 2007, neuroscientist Gabriele Jordan from Newcastle University in the UK, who had formerly worked alongside Mollon, decided to try a slightly different test to look for this super-vision.

She took 25 women who had a fourth type of cone cell, and put them in a dark room. Looking into a light device, three coloured circles of light flashed before these women's eyes.

To a trichromat, they all looked the same, but Jordan hypothesised that a true tetrachromat would be able to tell them apart thanks to the extra subtlety afforded to her by her fourth cone.

Incredibly, one of the women tested, cDa29, was able to differentiate the three different coloured circles in every single test.

"I was jumping up and down," Jordan told Veronique Greenwood from Discover magazine.

So if so many female children of colour blind men have four cones, why have we only been able to find one true tetrachromat?

For starters, the team was only looking within the UK. But the bigger issue that Jordan thinks most true tetrachromats would never need to use their fourth cone cell type, and so would never realise they had special vision.

"We now know tetrachromacy exists," she told Greenwood. "But we don’t know what allows someone to become functionally tetrachromatic, when most four-coned women aren’t."

Jay Neitz, a vision researcher at the University of Washington, who wasn't involved in the study, thinks that it might take practice and specially designed hues to truly unlock the power of tetrachromats.

"Most of the things that we see as coloured are manufactured by people who are trying to make colours that work for trichromats," he said. "It could be that our whole world is tuned to the world of the trichromat."

In other words, the colours we use are so limited that the fourth cone cell never gets a work out.

The research on cDa29 hasn't been peer-reviewed or published as yet, and Jordan is continuing her research and search for more tetrachromats.

There's a lot more work to be done and Jordan's results need to be replicated and verified. But if we can confirm that tetrachromats really do exist, it won't just teach us about the limitations of human senses, it could help scientists develop better artificial sensing devices, and also help us figure out more about how vision works.

One thing we might never be able to understand, sadly, is exactly what the world looks like through cDa29's eyes, seeing as it's our brains that truly perceive colour - our cone cells just receive the data to be processed.

"This private perception is what everybody is curious about," Jordan told Discover. "I would love to see that."