The blower will start with clear or some color they love.
If the blower is starting with clear and do it completely from scratch they will add minerals to the glass and mix it. This takes a long time to do. Many times glass sits in a crucible for days so the minerals can mix in the glass. There is also off-gassing from some of the minerals so it is best to do it in a well-ventalated area.
Many blowers will mix a few colors together making a color they want. This is done by just mixing and mixing by hand in the flame. This produces a small amount of one off-color.