There was a lady came to a site where I was working. Hungarian/Romanian and she brought some of her home grown and processed and hand spun hemp. It was beautiful. She graded it fine, everyday and coarse. Her fine was as fine as my hand spun and woven linen....and I've been doing it for a while, and so had she

It was white. Not dayglo white, but white.
We oohed and aaahed over each others spinning and weaving and there was not a word we had between us that the other could understand, but we knew what we were talking about

lots of expression and hand gestures.
She gave me some of her hand spun thread, and I gave her some of my hand spun St Kilda yarn and fleece.
I hadn't believed that it was possible to process and spin hemp so finely by hand, and she hadn't believed that it was possible to spin wool so finely until I showed her mine and the tartan I wove from it. She said that her fine was used for blouses, shirts and baby clothing, the everyday for tablewear, skirts, aprons, working shirts, pillowslips, etc., and the coarse for towels and sometimes in the past for sails.
My fine nettle fibres are pale greeny white at first but wash up white without bleaching. Linen wears white, eventually, but either dew or water retted, it's not white to start with. Cotton fibres are white, but the processing makes them filthy, and if they aren't careful about removing the seeds (the cotton gin was a game changer) then the resultant fabric always looks fly speckled. Bleaching makes a huge difference, and it comes up bright white very easily.
Bog cotton is white though you have to pick it before it discolours as the stems rot.