An orchid bloom opens slowly and rarely do we actually get to experience the process. Here is a time-release video of an orchid flower opening up to full bloom. The three sepals which make up the outer covering of the bud were green as the bud developed. As the bud opens, the three sepals fold back to show their lovely purple stripes. Inside the three sepals are the three petals, two large round petals to the left and right and one petal that has formed into a lip. On other flowers the sepals are not part of the bloom in the way they are with an orchid.
An orchid typically has three sepals and three petals, one of which is a lip. The picture of a Dendrobium below illustrates this. The sepals are white and the petals are purple. The lip, a special petal that forms into a unique shape, is also purple.