hair后面用is还是are

1、Her hair is going grey.她的头发日渐花白。

2、Her long black hair is her crowning glory.她长长的黑发是她的无上荣耀。

3、Her hair is a reddish-brown colour.她的头发是棕红色的。

4、As long as staff are smart， it does not matter how long their hair is员工只要聪明，头发留多长没什么关系。