If you are looking to print them onto what you have bought, then I would recommend using Lunapics, which is a free online photo editor, lots of options, you can even upload & then "pop art" a photo of your DS Andy Warhole style & then add text over the prow colourful picture - loads of other options on there too - sorry can't link from my iPad, but it comes up if you google it.
Definitely the rainbow food, we had a fruit course after the sandwiches and before rainbow cake, rainbow jelly and rainbow marshmellow krispies.
For the invites, could you make paper springs and stick different brightly coloured card balloon shapes inside the card, which pop out with the details when they open them (better if hand delivering than if you have to pay for postage).
I found some cute ideas on google after entering party invites for 4 year old and clicking on images.
What about blowing up a ballon and writing the info on with a permanent marker and then deflating it and sending that taped to your postcards?
When my ds was five we did a red birthday party because his favourite colour was red. We asked everyone to wear something red and everyone did and the kids loved it. You might consider doing something similar. the photos came out great and even some parents got in on the act.
I want to make my own invitations to da birthday party. He will be 4. We are having a kind of primary colours theme loosely - multicoloured bunting and balloons and the tablecloth and plates etc all different bright primary : rainbow colours.
I've bought those paperchase postcards and envelopes that are bright and multicoloured but now I'm stuck for inspiration.
I'm thinking printing or photocopying the written words and then stickers or printing or stamping but I've got no further. Can you think of a nice way to print and design them in a graphic designy way?