Here are a couple of pics from my daughter's rainbow themed birthday party.
It was a lot of fun and so easy to do. Lots of bright balloons and streamers... rainbow coloured fruits and lollies in separate little dishes... I also made rainbow jelly cups, which were a big hit and super yummy! (You just have to remember to plan ahead as you have to pour in each layer one at a time and then let it set before adding the next colour). I did the same layered effect with the birthday cake and stuck skittles all around the edge (which was quite time consuming but looked really effective). I made the invitations myself of course by starting with a hand-drawn rainbow and adding some colourful type.
(I have since turned the same rainbow into one of my Grubby Princess prints: over the rainbow).