Changing manga color can be done through traditional methods like using colored pencils or markers. But for more precise and editable results, digital tools are the way to go. You need to have a good understanding of color theory and shading to make it look great.
You can use digital painting software like Adobe Photoshop or Clip Studio Paint. Just select the area you want to color and apply the desired colors using various tools.
Well, one way is to scan the manga and import it into graphic design software. Then, use layers and color pickers to make the changes. It takes some practice and knowledge of the software's functions.