Charli XCX: Pop, punk and synaesthesia

Charli XCX - born Charlotte Emma Aitchison - has been performing on stage since she was 14

Charli XCX scored a number one earlier this year after writing Icona Pop's I Love It. The singer, who has synaesthesia, tells the BBC she gave the song away because it was the wrong colour.

Back in July, budding British pop star Charli XCX was poised to score her first ever number one.

The 21-year-old wrote and sang on Icona Pop's bratty pop ball-buster I Love It, but she couldn't bear to discover its fate on the Radio 1 countdown.

"I knew it might go to number one, and I didn't want to know," she says, "so I went to the cinema with my mum to see World War Z."

History does not record what she made of Brad Pitt's role in the zombie apocalypse film - but when she emerged from the multiplex, her phone lit up with congratulatory messages.

"It was really surreal to have a UK number one, especially when no-one really knows who you are," she says.

Icona Pop took I Love It to the top 10 in more than 18 countries

The song's performance was in stark contrast to that of her debut album, True Romance, which stalled at 85 in April.

A bold, brave collection of blitzkrieg pop, it had been delayed for a year - meaning fans had already heard, downloaded or bought the best tracks before it came out.

"I was working on that record for five years," she says, but by the time it was released in the UK, "I almost wanted to move on to the next one".

But she doesn't regret giving away I Love It, saying the moment she wrote it "I knew that I wasn't going to sing it."

The reason is unusual, but fascinating. The singer experiences music visually - with every song ascribed a colour - and she couldn't reconcile the "look" of I Love It to her sound.

It's a form of the medical condition synaesthesia, where the senses mix, meaning people can taste words, for example.

"I haven't been diagnosed with it but people mention it to me a lot," she says.

The singer supported Paramore on tour earlier this year, noting: "I fell off stage trying to be cool"

"I see music in colours. I love music that's black, pink, purple or red - but I hate music that's green, yellow or brown."

Drum and Bass is an example of the latter genre. The Cure's records, on the other hand, are "all midnight blue or black, but with twinkly pink stars and baby pink clouds floating around it".

"If I'm writing and I can't see what the video will look like in my head, I know the song isn't right for me," she adds.

And it works in reverse, too. Her new album is inspired by the colour palettes of the Serge Gainsbourg film Slogan, and cult 80s punk movie Ladies And Gentlemen, The Fabulous Stains.

She's also been listening to new wave bands like Bow Wow Wow and The Waitresses, as well as "ye-ye pop", the Francophone take on teen pop that flourished in France during the early 1960s, via acts such as Sylvie Vartan and Brigitte Bardot.

"Lots of girl power and lots of shouty, hooky choruses," she says. "It's quite punk."

The sugar-spun first single, SuperLove, deploys pop's secret weapon - the double handclap - and looks set to become her first solo hit in this weekend's charts.

"I began writing it a while ago," the singer says. "Actually, it was around the same time I wrote I Love It - and I left it for a while, because I didn't really think much of it."

"And then I went back and listened to it and I thought, 'oh, actually that's really cool'.

"It's about running away and falling in love, basically. It's seen as a bit cheesy to celebrate love but I just wanted to do that with this song."

With a playlist spot on Radio 1 (her first) and a number of high-profile media interviews, it feels like the singer is finally starting to arrive.

It's been a long journey from her first public performance, aged four - singing Barbie Girl a cappella on a cruise ship.

"The direction of my second record is definitely much more live… It's completely not electronic."

Back then, she was known as Charlotte Emma Aitchison. But by the time she was a teenager she had adopted her nom de pop and started recording an (unreleased) album, funded by a loan from her parents.

Some of her songs caught the attention of a club promoter and, aged 14, she started playing warehouse raves in East London - the first of which took place in a peanut factory.

Her early music was described as "nursery rhyme pop" with childish verses about dinosaurs and chocolate mousse.

Slowly, that evolved into something darker and more complex, squashing 80s chart melodies up against film samples and chugging, haunted synths to create a sound that was equal parts Madonna and Siouxsie Sioux.

Rolling Stone described the cut and paste aesthetic as the "pop-album equivalent of a wicked Tumblr" - but that was where Charli XCX seemed destined to languish, a so-called "blog act", stuck on the margins of the mainstream.

It's a description she bristles at.

"Even though I get classed as a 'blog act', I feel like I've done so many things that don't fit that label," she protests.

"I've played with Coldplay, I've supported Paramore, I've written a global number one for another artist.

"So even though I am a blog artist, I've played it very differently. I don't feel like I'm stuck in that world, and I can fly into anywhere that I want."

I Love It seems to have given her wings - alongside her second album, she's been asked to write songs for Britney Spears and Rita Ora.

Other offers came in, but not all of them were positive.

"It was like, 'we want you to write another I Love It for this artist and that artist,'" she says.

"But I will never write another song that sounds like that - because it was written in half an hour in a hotel room in Sweden without even thinking about it.

"I might write another hit as big as that, but it'll never sound like that."

The singer says a trip to Tokyo influenced her new record. "It's the future. It looks like the inside of my brain."

In fact, she's a prolific writer, often churning out three or four songs a day, mainly in conjunction with her "musical brother" Patrick Berger (Lana Del Rey, Robyn).

"We'll start at midday and keep going until 5am. We'll write until we can't write anymore."

"I like working really fast. The best ideas are the first ones that come into your head - so why bother thinking of any more?

"The whole idea of formulas and discipline freaks me out because I think it doesn't work. It has to be emotional and it has to be spontaneous. That's the best way to write a song."

And does she feel vindicated that the world is finally coming around to her way of thinking?

"I feel like I deserve it, because I've been working really hard for a really long time. So it feels great."

Super Love is out now. Charli XCX's new album is expected in June, 2014.