what color is the book ?的复数形式是什么

what color are these books?
what's the color of these books?

what color are the books?
color不用变复数的，因为是“这些书的颜色”这些书是一个整体。
the变成these变不变都没关系。
但是ARE 和BOOKS是一定要改的

就是这样。

what colors are these books?

what color is the book ?的复数形式是

what color are the books?这些书什么颜色？

补充：这个就是答案嘛

what colour are the books?
the可以修饰名词复数，不用改

what color are these books ?