If you don't use up all of the dyes at the time they are made up, how long can they be kept & used?
Thank you for your question.
As far as I know, the powder will keep indefinitely as long as they are in a cool dry place.
If the dye is made up with the vinegar I would say a few weeks, if made up without the vinegar added then a lot longer.
This, of course, depends on the storage conditions.
Does the kit come with instructions/formula for achieving a spectrum of colours from the three primary colours?
Thank you for your question,
Yes, the kit comes with everything you need including instructions and formulas to create the desired colours.