What a very expressive design! The colour, an clothing style and accessories all help build to a fun, carefree music-loving character. I would say be more consistent with the line work. Some areas have bold dark line art while other areas (excluding the shading) have soft to no line art. If you are working with an art program that provides layers, separate the line art into its own layer. If not, you can always go over the painting again with lineart

These princess designs looks so elegant! Often the color helps to compliment or build onto the characters personality, their role in the story or some other story telling method. I think it depends on her role in the story. If she is bold, I would choose something like red (often associated with passion). Purple is also associated with royalty (due to its rarity). I would also choose white if they are considered on the "good" side (but colors can often be used to mislead the viewers)

These concepts are so amazing! !! They're very prominent with their mood. Did you design these for just the films or were they also for games, posters and others? (Would you say the design process change depending on what medium you were making concept for) also what would you say is the key to creating well designed sci-fi armor (I especially love the yellow captain marvel? ones)