Purple areas indicate which parts you will be able to apply your own texture to.
If the combination you choose already exists, Stylemaker will find the mesh for you and allow you to derive right away.
Design note: make sure the client is shut down and not running in the background when doing this.
Additionally, files automatically downloaded by IMVU are referenced by their PID - "Product 123456" for example - if the project needs to be or is edited at some point in the future the Client will download the files again as necessary.
Using the IMVU previewer, meshers can add their own meshes to Stylemaker derivations.
You are most likely to see this issue with enhanced or 'hippy' pelvises, which are likely to clip regular skirts.