Seems a daft question but living alone I've always put light coloured clothing in with my whites to make a full load. I've now seen white patches on my light blue towels and am wondering if all coloured materials should be washed with laundry liquids for coloureds, no matter how pale.
My sister in law once said that whites turned grey if washed with anything coloured. So since then I have always washed them separately even if it means waiting a while to have enough to make it worth running the machine or even running it with very little in there.
I can't see how white washing would bleach out coloured fabrics simply by sitting next to them in the machine, though.
I think biological powders have optical brighteners in them and oxygen based bleaching agents so it’s probably safer to wash any non-whites in non bio or stuff for coloureds.
The vast majority of my clothes are pastel and I use those 'Colour Catcher' sheets which seem to help. I've not noticed any strange patches so hope they will help for you.