>>178580manual monochrome shading + gradient maps + correction layers on top + lighten blending mode on merged colors.
gradient maps basically let you turn the full range of a monochrome image into certain colors. so you select an area, you apply a gradient map locked to that area, adjust your colors and you get a result like this. some monochrome images are not made for gradient maps, the more tones you have the better, so you can add depth and lighting. if it's too cel shaded at that point just use the "color" blending mode on another layer, or color below it setting the "lineart" layer to multiply.