I'm new to Krita. How to make gray scale for colored image?

Filter -> Adjust -> Desaturate -> choose one of the 'Luminance' filters, whichever you like more (it's more true to our eyes than "Lightness" which is the default method in different art programs).

You can also add a new layer on top, fill it in full black and change the blending mode to color it will do the trick