Should i let my granddaughter dye her hair?

My granddaughter lives with me and is 11 and wants to dye her hair purple, pink, blue or green i told her its ok but i don't know if she should she said she's going to dye it soon then dye it again for her birthday and then again for the summer i dye my hair once a year and her mother dyed her hair when she was 16-18 so should i let her? is it ok? she wheres hair extensions and use's hair spray and straightens her hair a lot so yeah....