why has my rooster comb go abit purple???

It could be stress, lack of oxygen,s omething to do with the feed? Have you changed your feed lately or the number of your birds?

Did the weather change maybe? Or did he eat something wrong? I had a hen do that to me and she came down with an impacted crop shortly thereafter. Maybe give them some yoghurt and ACV to flush things out and then some nice treats like watermelon? Just a guess

I see this happen to my roosters once in a while. the color eventually comes back. I don't know what causes it. My birds have it good. They free range and they have access to fresh food and water all day.

I think its because they are keen to mate! 

I'd treat him for upper respiratory with some antibiotics.  Does the face turn purplish blue too?  Be careful not to stress him out in any way.  Don't lay him on his back, things like that.  Make sure coop is free of ammonia smell.  Make sure he's eating.

Yeah, what is the weather like in Ireland? Did the temps get a bit cooler? Happens all the time to my roosters when there is a sudden change of temp. If its temp related, its absolutely normal. Their blood flow goes to more important places for survival,

Comb color is an imperfect indicator of health in chickens.  Sometimes color changes mean something pertinent to a bird's health, but often it means nothing.  Temperature changes, hormonal changes, humidity changes, light exposure, and bird health are all things that could lead to comb color changes.  If your bird is lacking any significant indicators of health problems, then I would just watch him, but not worry too much about him.

Good luck.

mine has done the same but i dont know what too do i have read that the best thing to do is isolate the cockerel keep it warm but not hot and feed and water it clean fresh food and water.