Ok,the number of colors, must see to use more than 3 colors per color for leftovers ... I use the pencil to do the drawing and look for forums and say that the ant-aliasing is turned off in pencil ... thanks for your help

this seems to be the right size now, but has tons of colors, which is probably due to the brushes automatically anti-aliasing when you place pixels. if you place a black pixel on a white background, the computer will probably add grey pixels around it if anti-aliasing is on.

on a formal level, once that technical issue is worked out, I'd recommend picking a strong lightsource, using maybe 3 shades of green (dark, medium, light) and 3 shades of brown to produce areas of highlight and shadow, based on the direction of your lightsource.