Sudden switch of colour during print (Not multi material)
I was printing a white storage box but when I went to grab the print afterwards, I saw a gray stripe going through the middle of the print. 
It looks like a filament I have used before but that was one entire roll (1kg) before this print.
Another problem with colour I’ve been getting is these brown splotches on ear the bottom of the print. I check the PEI board before printing yet it keeps appearing.
Definitely not a filament I’ve used.
RE: Sudden switch of colour during print (Not multi material)
The discoloured band might be dirty filament - I have seen similar discolouration when a certan young lady spilled a milk drink (I think) onto a spool without telling me. In that case the smell gave it away and I aborted the print.
The blotches might be dirt on the build plate or excess burned filament dropping from the extruder. When in doubt clean the plate and check the extruder - if there is an oily looking dark residue preheat and clean it with a fine wire brush.
Cheerio,