When two sisters launched an "anti-pink" campaign two years ago to liberate girls from a toy industry dedicated to churning out pretty princesses for girls, they had no idea of the fuss it would cause.

"We got hate mail from all over the world," says Emma Moore, one half of Pinkstinks, the group she runs with her twin sister, Abi. "They said things like 'you must be lesbians, you're ugly'. The reaction was so extreme you'd think we'd tried to cancel Christmas."

Notwithstanding the backlash, the sisters have turned out to be at the vanguard of a movement whose time may have come.

When the London toy store Hamleys stopped labelling its floors in blue for boys and pink for girls last week and rearranged toys by type rather than gender, there were loud cheers from those who believe the pre-teen pink-blue divide has gone too far.

A campaign on Twitter had accused the store of "gender apartheid", but although Hamleys denies that the initiative was a response to that campaign, the relabelling was claimed as another small victory in the campaign against the "pinkification" of girlhood.

Before the rejig, Hamleys' girls department was home to a menagerie of fluffy animals, cookery sets and hair-and-beauty paraphernalia, including a beauty salon called Tantrum, while the boys floor was a Bear Grylls-fest of cars, spaceships and construction sets.

The American writer Peggy Orenstein calls this an example of the "princess industrial complex", arguing that in the late 1980s marketeers realised that there were bigger sales opportunities if the toy market was divided into two distinct camps.

"Colour didn't come into the nursery until around 1900 and, when it did, pink was for boys and blue was for girls," says the author of Cinderella Ate My Daughter: Dispatches from the Front Lines of the New Girlie-Girl Culture. "This profound fetishism of pink is very recent and is a market-driven construct. Princess is the only game in town."

Pinkstinks argues that the toy industry increasingly makes products for girls that centre on being pretty, passive and obsessed with shopping, fashion and makeup. Its second, smaller campaign successfully challenged Sainsbury's over the labelling of children's dressing-up clothes, whereby doctors' outfits were labelled as being intended for boys and nurses' and beauticians' outfits were tagged "girl". The end point, they say, is a narrowing of girls' choices and aspirations.

"Girls are increasingly worried about the way they look, and feel like they can't get ahead in their lives unless they look like Cheryl Cole," said Moore, who has two daughters aged five and nine. The sisters founded the group after having children and becoming alarmed by gender-segregated toys as Emma's house filled up with pink plastic while Abi's — with two boys — became packed with Power Rangers and the like. Moore added: "That's not an issue for every single girl but is an issue for a significant number of girls."

There appears to be little scientific evidence behind the colour coding used in toy shops. A wealth of evidence indicates that colour preferences are learned rather than innate. Some studies have suggested that boys are pre-programmed for rough-and-tumble games and toys with moving parts, and that girls are drawn to dolls and role-play, but the evidence is not conclusive.

Pinkstinks, which was founded in 2008, is run from the 40-year-old sisters' respective bedrooms in Lewisham. They are supported by a small group of volunteers who help decide the campaign strategy, which relies on social networking, video and blogging to raise awareness and tackle companies.

So far they have gathered 5,000 followers on Twitter and some 2,500 supporters on Facebook.

"It is quite exhausting sometimes," said Moore, whose day job is as a project manager for a research firm while her sister is a freelance film-maker. "We try to raise money by selling merchandise. If we sell a T-shirt we raise about £2." They visit schools to give talks and despite their size frequently receive requests for "teaching packs".

The next campaign "slap on the face of childhood" will tackle the growing market for makeup aimed at young girls.

Gary Grant, managing director of The Entertainer toy chain, said: "If girls didn't want pink, we wouldn't make it pink." The bestselling babywalker on the market was originally only available in blue, but when a pink version was introduced it outsold the original, he said.

"Is that the manufacturer manipulating the market or consumers deciding what to buy?" he said. Lego was predominantly played with by boys, he claimed, with attempts by the manufacturer to get girls on side with pink bricks an "unmitigated disaster".

Pinkstinks's first campaign targeted "sexist" toys, including a pink globe with mermaids swimming in the sea, stocked by the Early Learning Centre. Within two weeks it had gone global and reached 45 countries. However, not all of coverage was positive. One newspaper branding the Moore sisters "dour and humourless feminists".

The extreme reaction to the first Pinkstinks campaign led the sisters to question whether they should carry on. "We don't make a living from this, we do it in our spare time," said Moore. What swung it in the end was the steady flow of emails they receive from young girls. "They make me cry. They say we need to carry on, you're our voice." Increasingly, judging by last week, it is one that is being heard.