A great way to theme your wedding is through colour.Flowers provide an excellent way to colour coordinate your wedding and the variety of colours available is amazing.
If you have a strong colour like the yellow you can see here from http://www.wedding-flowers-guide.com/yellow-wedding-flowers.html it enables you to dress all of your rooms from the ceremony to the wedding breakfast etc.
Often brides use the colour of their bridesmaids dresses as a starting point.
When you have a colour theme in mind I suggest talking it through with your florist. They will be able to advise and help you coordinate your colour theme through the flowers.
This photograph is of a spring wedding and the fresh yellow colour worked perfectly for the time of year and the setting.
