Sorry for the late reply, I forgot about this post.
My method.
Well firstly, I honestly don't believe the dark brown colour is from the stout, I have tried and tried and the colour of the stout hardly absorbs into the curds.
So, well I wont go into huge details as the method is the same for any farmhouse cheddar.
The only difference is, I split the milk into 2 separate bowls, and on 1 added brown colouring (to the milk, not the curds). So I ended up with 2 bowls of curds, one brown one white.
As for the stout, I reduced it down by about 50%, then added it to the brown curds after draining and gave it a good mix before salting.
To be honest about the flavour, I think I ruined it by over salting. Its a shame but it was more of an experiment on the method rather than the taste.
If you look at Cahill's Porter Cheese, it seems to me they are using chopped up cheddar for the white. So next time I am going to try and make a 30% volume of dyed milk curds to 70% volume of chopped cheddar then try and repress it...