What an awesome and effective icing technique! I don't have any photos of the process, but its really easy to explain.
I used Gel colours for this as liquid colours would run. I placed my piping bag into a glass and opened it up wide and folded it back over the outside of the glass (so the bag is wide open). Then using a paintbrush I just painted the insides of the piping bag with gel colour, added plain uncoloured butter icing and piped.
They were so bright and the kids were fascinated with them.