Moutaz,
In P3 you will select all activities by hightlight functions and click View-> Format bar to define new bar colour.

In P6 you will select all activities by creating filter and click View-> Bar to define new bar colour.
It is very simple steps as stated below:
1) First Create a Filter (ex: WBS under ????[select the WBS that requires bar formatting] and name the filter like WBS ???? bar format)
2) Click View-> Bar
3) Click Add and name it WBS ????
4) Display column should be automatically tick
5) Select the timescale for new Bar (i.e. Remaining Bar or Current Bar)
6) Select the filter that you created (i.e. WBS ???? bar format)
7) Define the colour, size, end point and position of new bar.
8) Click Apply and you will see that all activities under that WBS have new bar colour.