Continue from last drawing tutorial "Easy Way To Draw Pikachu"
First of all use a lineart layer like this, normal layer could work but lineart layer will be easier for these stuff.
Select magic wand , a wand like tool which is selected on my screenshot. Then Click somewhere inside lineart when selecting lineart layer. Their color should turn blue, Which means every brush will only affect these selected part
Pick a color on color pallets and select your own prefered brushes, for color reference you can see google for one. To make accurate color open picture in paintool SAI, And then use ALT + Left click to pick a color from other picture, make sure you do it in intended colored part.
Repeat Previous step until all part are colored.
Make new layer, i usualy lower the opacity for a very quick coloring.
Color it around these parts (or other you wanted) dont worry about messy look will be fixed in next part.
In this part use eraser slowly and shape the shading into a better look. Then repeat same things to every desired part to shade.
Voila,.. And it is done. A full colored pikachu.
For any Question just ask me, i will try to help you to find out.