Hum bit late for crocuses though if you could hold them back it might work. The snag when you want anything for a specific day is getting it to flower at the right time. Commercial growers keep plants in what are, effectively, refrigerators and bring them out at just the right time to be in bloom for the event or function. They also accept a huge wastage.

Hi, the majority of spring flowers tend to be yellow, as this is about the easiest colour for a plant to produce in the growth time they have for spring flowering, Derek.

Purple freesias would be available, but expensive in quantity.

Some of the Iris reticulate are purple but they tend to flower in March rather than April.

Hi try this site -- www.flowers.org.uk/flowers/flowers-by-month/april/ - they have a list as long as your arm - failing that you could use Artificial ones (joke) -- Ken

It might be a good idea to speak to a florist though, as a fail safe. They seem to be able to get all colours at every time of year and may have blue irises, which is the nearest I can think of. Its always dodgy trying to grow your own for a wedding when you have particular requirements as there are so many variables and its difficult to be sure of getting the timing right.

Check out foliage too, Strobilanthes and Oxalis have great purple-leaved varieties which are sold as house-plants. Also in the same section in the garden centre, African violets and achimines come in good purples.
If you can source them locally grown rather than through a florist, you could task your wedding guests to help you accommodate them and grow them on over the next few months, as a project!