How do I achieve this deep purple BC frosting on these cupcakes without making the BC taste bitter? Once I start adding lots of colour to make BC dark I get a horrible bitter taste. This is for someone's wedding and I don't want guests to have a bitter frosting.

Pearl, you do not colour the entire batch that deep shade.

You colour the icing to a medium shade and let sit overnight. Then you take a clean brush and brush paste colour all over the inside of your piping bag. When the icing is added, the deepest shade is on the outside surface and won't be tasted.

This technique is in the Wilton books pre-1990. People sell the Wilton Celebrate hardback books (the oldies but goodies) on ebay for very little $$. Consider picking up one or two, there is a lot of useful info. The patterns have all been posted online by Wilton.

Excellent idea Irene.

Another option would be to *flavor* & color the icing by adding about 1/4 teaspoon (per cup of icing) of KoolAid UNsweetened powder to a good medium shade as noted in the other posters suggestion. this also would need to sit overnight. Pick whatever flavor of KoolAid that is blue or purple.
Just one caution regarding the other suggestion. It will *stain!* everyone's lips/teeth & any fingers that touch the icing

Okay great. Sounds like a plan. Will give this a test run. Thanks for the responses.