If you don’t want chemical colourings, try some of these natural alternatives. To get the most colour from the foodstuff, take the food (such as a handful of raspberries) and simmer them in liquid until reduced, then strain and use the liquid. The juice for things like blueberries can be brushed over set icing with a pastry brush.
Turmeric (Use sparingly in sweeter foods as the taste may overpower)
Beetroot (Buy beetroot, slice it and simmer it in water, or buy canned)
Purple grape juice
Freshly juiced spinach
Spirulina (an algea herbal supplement)