alright so we're gonna make a skin file with a solid colour for the hair
kind of like this:

the way we'll be shading the hair is using the custom brush feature on planet minecraft skin editor

first we'll import the file i personally use for my skins' hair, which you can download below:
<--- very small

to import the file, just click on the three dots then "import image as brush" and navigate to the small png you just downloaded

first, select the brush you've just imported. then, change the opacity to around 13%
then you'll want to put the brush over the bit of hair you're texturing (an 8x8 square) and click. you can layer it or just click once. it's up to you

a tip for making more vibrantly coloured hair is to start with a base that is very, very, very bright. this can be seen in the image below - the left was a normal shade, the right was a very bright shade

finally, if you're having trouble with the rotation, you can always change the x and y axis as well :)

if you want to make your own textures, use black and white to mark out the texture. then copy it with the copy tool and and change the opacity. you can save what you copied as a brush too!