Do you know very much about traditional British dyes, moss, as I'd like to learn more..

Used to spin years ago, and also dyeing wool with plants. A lot depends on what soil plants are grown in, and at what time of year they are picked.. Best book is Jill Goodwin, A Dyers Manual, lots of native dye plants, lichens and a whole chapter on woad.
Colour whether in beads, or clothes, or on stones, would have been there. Today we just see gray stones but decoration of body/home/totems whatever would have gone back to the earliest time.
On the chalklands of Wiltshire, I supposes white represented death, you would become aware of it as the long cursuses were dug and also the longbarrows, it would become a symbolic colour on the later barrows - just as black represents death today...... which, if Silbury was indeed covered in chalk would make it a very large barrow..