hey man. i guess its kinda intuitive, its a bit more random than that i think. it requires planning to a small degree, before i start i always make sure the drawing i have down first is perfect, obvious i know but i' ll explain why its especially important. then i grab some pure colours, like a true red (ie not slightly pink or brown), maybe about three colours, i like yellow red and blue. when it comes to actually painting i always decide before what areas of the face are going to be primarily what colours. i find t really helps to print off the photo im working from in greyscale so im not influenced in anyway. i guess deciding what areas are going to be blue, red or yellow is somewhat intuitive, i tend to make the areas that catch the light primarily yellow or red and areas that are in shadow i seem to paint blue or green. to start painting i lay down colours straight from the pot onto the canvas, i don't ever mix the colours at this stage. because i hate washing my brush, im a little lazy, i tend to find myself painting kind of 'locally', and by this i mean i focus on small parts of the painting each at a time rather than giving my attention to the whole work. this is why i say i make sure i plan it out perfectly, or as accurately as i can, because if i dont i find that almost always i focus intensely on an eye for half an hour or more only to realise some time later after i have picked up the next colour and washed my brush and i go to paint the jaw line its all wrong and i have to shift stuff which is always more annoying later in a work. so once ive put down the first sort of base layer of colour i work each colour separately until im happy with it. so i might sculpt out the nose and an eye in red first before moving onto yellow cheeks. thats pretty much the gist of my process only when im very close to finishing a portrait do i mix my colours, i might make corrections in a red area with blue or green, but this i almost always the very last step that helps tie the work together. hope this helps you understand how i do it. if you want to give something like this a go go buy yourself some bold colours, a red yellow and blue maybe. get yourself a couple of pallets and maybe a few brushes for each colour so you dont mix them. plan out the face really well, i find the bigger i paint the better results i have. work from a greyscale image. try not to tint or shade your colurs too much, and try not to blend your colours either, paint solid areas of colour dont blend on the canvas, i like to think of it as im pixelating the image, im reducing the resolution to the size of my brush, if you look close at my work its just a jigsaw of pure colours, there isn't a lot of blending but when you stand back the human eye will blend all those areas together just like when step back from a heavily pixelated image all the pixels seem to merge and an image is found.