Waitrose and Tesco no longer seem to stock the food dyes (Langdales) we usually use for these experiments – they’ve replaced them with Dr. Oetker dyes that doesn’t seem to work at all, because they're a suspension of insoluble natural colourings; they're presumably OK for cooking, but they're pretty much useless for this experiment.
Can anyone suggest alternatives, preferably in several colours? Or point me at a source for the Langdale colours or something like them? We need something non-toxic (which hopefully applies to all food dyes), at least two or three colours - it would be useful to have red, blue, green and yellow - and non-toxic to plants, which rules out some of the chemicals I might otherwise use. Someone has suggested dyes used by soap makers, any other ideas?