Pink being a girls thing is a capitalist agenda to sell the same products with a different coat to more people and is a relatively new thing.
They started it by saying that since (at the time) pink is a color that is very brass and strong like red it was a man’s color (whatever that means) that if a woman were to wear them it would mean they were empowered because they were wearing a man’s color. Trying to cash in on the female empowerment movement of the early 20th century.
Other industries (especially children related industry) followed interest on what was now a bigger market because they can sell more stuff if they said x color was for x gender. -_- its really disgusting when you think about it.