If not then it's best to let that one pass by. There will always be another similar swap later that you can participate in. I've seen swaps that had huge numbers of signups and then many drop-outs occurring at the last minute. Some of those hostesses were so discouraged when this happened to them that they never hosted another swap. We all have things happen in our lives that are out of our control.
Hostesses understand that.
Just be courteous and let her know if you need to drop out. It makes your blocks nicer and is always a plus with the hostess to see that you care enough about your work to invest a little more money in the fabric you use. When I plan a swap, the first thing I determine is the theme of the swap. Then I decide what color scheme I want to be used. If your hostess has stated a color preference, it is your job to make sure you follow that color scheme.