The girl always says she wants to see a girl in a colourful outfit but is there a specific outfit you gotta wear?
For female characters, you will need pale pink coloured clothes. You need to wear at least 3 accessories that are of the pale pink colour.
I tried with 2 pale pink accessories and everything else pink but it didn't work, whence my conclusion above.
I also tried 3 pale pink accessories, with everything else being another colour; black, grey, etc and still got the accessory.