Ideas for a rainbow coloured late morning wedding in September.?

We want to use pastel colours like pink, blue, green, yellow, purple & peach but don't know how to combine it all together without looking like a birthday party. For flowers we thought of roses and tullips. Decorations with lot's of gl and mirror's. We want to keep it cly, traditional, fun, romantic & casual. Our budget is a bit tight as well. My head is spinnng with ideas & i'm getting a bit nervous for our big day! Any ideas would be helpfull & highly appreciated!