Most common 2D concept art pipeline
I start drawing basic black/white sketches on paper to get the "style
We select the best sketches and I then do a colored speed-painting.
When everything feels 100% I do the high detailed illustration.
This is process is recommended for any high quality productions
where you want your interface design
to have that little extra perfection.
You can always ask to have a basic sketch or a full illustration from the start if you prefer :)