How to prevent food colouring changing the taste of food?
If you use the same Dr. Oetker colouring that I have used in the past, which is advertised as a 'gel' colouring, then you have to use a lot to get a good intense colour.

You may have better results (colour and taste-wise) getting proper concentrated gel food colourings, like those made by Wilton. You need to use much less than cheaper brands so you don't run the risk of flavouring the food, and you can even use them in meringues without ruining the structure.