First a basecoat of 1:1 mix of khorne red and abaddon black, then I mixed the same colours with extra black, added extra water and washed this in the recesses. Here the important thing is just to cover the recesses, if it goes over onto to surrounding flesh - no problem as this isn't a neat stage.
Next I used a 2:1 mix of khorne red to abaddon black, I used a medium sized brush and added only a bit of water to the mix so it flows but also stays controlled. I applied this mix everywhere other than the recesses, which leaves dark lines in-between to define the muscles. If you do get this colour in the recesses between muscles, correct it using the previous wash mix.
After this I started the first of two highlight layers, I used pure khorne red and painted this on the upper most areas and edges of muscles, try to leave some of the previous colour showing through. Any mistakes can be corrected with using one of the previous mixes.
The final highlight is blood red from the old GW paint range (evil sunz scarlet in the new range is equivalent). I thinned this with some lahmian medium to stop the highlight effect from being too stark for the flesh, then applied this in thin lines around the outermost edges of muscles. After this I went back for a fair amount of correction with khorne red.
Lastly I painted the bumps of the daemons skin, for most of them I just used lugganath orange, except on larger areas such as the back, for the lower half I used lugganath orange mixed with a bit of red, and then pure lugganath orange for the upper half. I did this to try to show how light would fall on the model from above.
I'm not sure how clear this was about placing colours, so although this looks dodgy compared to on a model, it should illustrate where I would place the various colours on a muscle:
So that is how I painted the skin on that bloodletter, although this same technique with different colours could be used for painting any colour skin, particularly high contrast non human skin like Orks!