Purple water from hot feeds

[roadhousezamora]

Hi, I've had a quick search and can't find anything similar so I thought I'd post this quick question. I've had a plumber install a hot and cold feed upstairs to my new bathroom. I've fitted the bath and sink mixer taps and the rainfall shower head.

When I have a shower and I come back to it the next day there are purple water stains left in the bottom of the bath from the rainfall shower head. When I come to run the hot tap on the sink it runs slightly purple for half a second just after I turn the tap on if it's been left unturned for a day or two.

I've read this may be flux in the pipes or possible potassium permanganate from somewhere but ultimately I don't have a clue.
The water feeds are a lead main feed in attached to a short length of copper. This changes to PEX pipe up to the bath sink and toilet where it changes back to copper to feed into all 3 items. The feeds to the boiler are PEX upto a meter from the boiler where everything is again copper. All copper joins are soldered and PEX joins are push fit.

I've attached a pic showing how the bath ends up although this doesn't really show how bad it can get and I can add more pics later if that helps.