Angie, I think it has also to do with the character itself, like the way she is portrait. Anyways, it's funny how that seems to be the 'imaginary' for preppy on Spanish blogs. I never thought of khakis as 'preppy' before! To me, it conveys posh+polished+young adult (kids looking like adults)+college.
Then again, we don't even have a Spanish term for that, so I'm probably wrong in my understanding!
PS: "En cuanto a la mujer, el preppy les sienta como traje de “niña buena”, lo cual puede ser muy clásico y a la vez muy sensual. Es así que para las fanáticas de esta moda va muy bien una falda de tipo colegiala abotonada, con camisa blanca y blazer entallado; en los pies, bailarinas o tacones. Otra opción puede ser una camiseta marinera, con chaqueta, pantalón beige, tipo náutico y zapatos de punta redonda."
Translates into:
"As for women, preppy style looks on them like a little girl's outdit, which makes her look classic and sensual at the same time. A school uniform skirt -tartan- buttoned up with a white shirt and a structured blazer; ballet flats or court heels for the feet. Another option could be a navy/sailor inspired shirt with a jacket, beige trousers and round toe shoes"