Hi
Milky pees are the natural way of guinea pigs to excrete any excess calcium. The brown comes from the soaked poo that has nothing wrong with it; it looks like a normal boar poo to me.
Calcium pees can sometimes herald the very first stages of a developing urinary tract infection but it will take several days more until symptoms manifest consistently. Screaming when peeing or pooing can also happen with bladder stones/sludge or with problems in the surrounding areas (penis shaft; impaction or arthritis in older boars) .
I would strongly recommend to keep an eye on your boy and have him booked for a vet check during regular opening hours when you notice regular wheeking, porphyrine coloured pees that look like blood, straining or general discomfort.
You may find these guides here helpful in learning what is normal and what not:
Guinea pig body quirks - What is normal and what not?
Boar Care: Bits, Bums & Baths
Weight - Monitoring and Management
How Soon Should My Guinea Pig See A Vet? - A Quick Guide
Emergency, Crisis and Bridging Care until a Vet Appointment