I'm awful at explaining things but this might help

1. Start with a soft brush with 'sharp' or softer edges to paint out the mane, always start with a darker colour and obviously add more highlights to make the mane brighter if needed depending on what colour your going for.

Never use black or white when it comes to manes, you can use similiar colours for greyscale but never the actual black or white colour.

2. Use a smaller brush to start with the highlights, look at the original stock for a possible idea of where they are placed and depending on how light or dark you want the end result you can alts r the brightness of the brush.

3. Use an even smaller brush to add more and brighter highlights, it's better to use these highlights in between the other ones to get as much detail in as possible.

4. Add shadows using a darker colour than the original layer, these aren't for the lightning of the background so kuch you can add that in later (that's what I do) these shadows are for the hair clumps, for example on the forlock there was a shadow because some hair falls over the other

5. Here we can add even lighter and smaller highlights. Once again in between the lines, to add more detail

6. This is simply adding shadow to the hair to add realism