i have a problem with my photoshop. why is it i cannot change my foreground and background color every time i press d and use paint bucket its ok even if press x, but if i choose other colors like red, blue the colors turn to gray or black! what does this mean?

Sounds like you're working in grayscale mode.

so how do i change back to rgb mode? how did i get to gray mode?

Image>Mode>RGB

Your image mode is displayed on the title bar of your working image window.

As for how you got into GS mode, I don't know exactly. Perhaps your last image was in GS, or you performed a function that converts to GS.