I have not done a cake in Years and have been asked to do a Wedding cake  for a Family member . I decided to do a test run and it did not go well . I used the crusting butter cream and it was great . crusted perfect  until I put the Wilton gel coloring in. The cake needs to be a Royal purple . By the time I got enough Gel color to make it as dark as it needed to be  the Icing was way to soft and will not crust . so I can't smooth the cake or paint on it . Any Suggestions ? 

You might want to try coloring the icing while you are making it. Replace some of the liquid with the food coloring. Also, you might just want to do fondant. Dark colored icing will stain. 

I thought about fondant but I am old School and don't know if I can master it quick enough . The wedding is june 4th . if I do try it then I will have to make My own . the store bought taste nasty . that was another issue I had with the royal purple butter cream it tasted like soap from all of the  gel coloring .

Have you thought about starting with a lighter purple base then adding additional royal purple to get the deeper colour.  I recently had to make navy blue and started with royal blue, then added some navy to get the deeper colour.

Try powder food colors instead of the gels.

I just made a deep purple cake this weekend and be aware that dark colors give the buttercream a funny taste.  I made the cake 4 layers so I could put most of the buttercream as a filling since it did not have any gel color so I could remove most of the dark buttercream as we served the cake.

The town where I live and the one next to us both have royal purple as one of their school colors and somehow I always end up making several graduation cakes every summer. What I do is color the icing two days before I want to use it and then re-beat it before I use it. It always gets darker when it sits for a while. Then I cover the cake with white buttercream and put just a very thin layer of the purple over it so the taste of the purple isn't so noticeable.