how to I change gray line drawing into color?

how to I change gray line drawing into color?

Thanks

Image - Image mode - CMYK or RGB or Indexed or.....etc

Then you can use Hue/Saturation with selections or can make layers with color fills set to blending modes such as hue or color.

There really is a million ways to do this but the first step is always to convert the entire file to a color format.