Help - first time cake decorator now thinking I can't do what I want to do!

Hi there!

A friend has had to let me down with making a cake for my friends 50th… she couldn’t help it and I understand and I thought I’d rise to the challenge! I came up with my basic idea, which was decorating my cake in psychedelic colours to match our 1960/70s theme for the party. I’ve bought my deep purple fondant icing and I wanted to write all over the cake, word cloud-esque, band names, music genres and festivals that mean something to the four recipients of the cake. I’ve looked up and seen you can purchase edible ink icing pens and I thought perfect! But then I wondered whether the yellow, orange, red and green pens would actually show up on the deep purple fondant or whether they’d just look dark against the purple! I’m running out of time and don’t have chance to practice different things so I was hoping someone might have any ideas as to whether the colours will show, or is there an alternative I should be using and how easy are they to use.
Many thanks in advance,
Sam