When I use Photoshop to add black color on a armor texture It becomes black and white or a bit gray.If you have tried it you would know how it changes.So I want to add a black color on the texture to make the armor black but I am not able to do that what should I do?Can someone help me.

Turn the Opactiy down to 80% and turn the mode to normal.

Just select the part you want to paint, then right click inside the selection and choose the Fill option, then select the background color (previously set it to black, in the tools window) ... later in the Blending field choose normal, and in the Opacity field set 100% for full black or the fade you want ... (all this at least in my Photoshop 7.0 version) :)
This way you will FILL the selection ... maybe you want only to DARKEN OR CHANGE the color in that case use the Image/adjustments options available in the upper menus.

good luck. ;)

Well the way I usually color black in photoshop is to do what you did first, make it grayscaled by using the "color" option with a black brush. Then I switch to the "overlay" option with a black brush. For less black just turn down the opacity. But see, if you try and overlay black before making it gray first it will still be the tint of whatever color it was before.