The colouring should not be affecting the taste. You need fat soluble coluring, not water soluble colourings. I have only ever seen fat soluble colours in powder form. If adding colour to white chocolate, you can temper before or after you add the colour, just make sure the chocolate is in temper before painting your moulds.
PS: I often use coloured white chocolate to give some colours a better pop than cocoa butter on the finished product.