Color war is fine for summer camp, but when it comes to children’s books, there are plenty of people who don’t want to have to choose between blue and pink.

Timed to World Book Day, which the U.K. and Ireland observe more than a month before the rest of the world, the U.K.-based Let Toys Be Toys campaign – which has advocated for stores to stop labeling playthings as “for boys” or “for girls” – has begun a similar initiative called Let Books Be Books, aimed at erasing gender stereotypes from children’s publishing.

By the end of launch day on March 6, an online petition encouraging publishers to “stop labeling books [in a gendered] way and let children decide for themselves what kinds of stories and activity books they find interesting” had garnered more than 1,500 signatures, most from the U.K. but also extending to continental Europe, North America, and beyond. A Twitter hashtag invited users to join the conversation on stereotyping in children’s books; one parent tweeted: “daughter was excluded by classmates today as not dressed as a princess for #WorldBookDay Sad #LetBooksBeBooks.”

Some of the most eloquent discourse, though, is on the petition site itself, which gives signees the opportunity to explain why the cause is important to them. Kate Francis in Washington, D.C., wrote, “As a mom of boy/girl twins, I strive to support my daughter’s love of Thomas the Train and my son’s desire to wear barrettes and tutus because the idea of limiting their explorations of who they are is not beneficial for them or society as a whole. When girls represent a tiny fraction of science-oriented professionals and boys’ images of masculinity are one dimensional, society loses.”

Gemma Atkinson in Tartu, Estonia, referenced the titles shown on the petition page: “I am a woman scientist with a little daughter. I find it offensive that the “boys’ ” book is clearly associated with academic achievement (“brilliant”), science and technology, while the “girls’ ” book is associated with frivolous, superficial things. Disgusting.”

And Monica Boyd of Tempe, Ariz., summed up her reasons succinctly: “Rigid gender roles hurt children by limiting their possibilities for a full life.”