Imtheperson97:
if string.h in the arduino but why the string.h words still in black colour like #include<string.h>
Because the Arduino editor is relatively dumb. Every library you have installed can tell it to highlight certain words. But it doesn't look at which libraries you are actually using - those words are highlighted even if you aren't using that library.
The syntax highlighting is useful if you've mispelled for() as For() but beyond that, I ignore it.