To find out how much more blue paint Juanita needs, we need to subtract the amount of blue paint she currently has from the amount she requires.
She needs \( \frac{7}{8} \) gallon of blue paint and currently has \( \frac{5}{8} \) gallon:
\[ \frac{7}{8} - \frac{5}{8} = \frac{7 - 5}{8} = \frac{2}{8} = \frac{1}{4} \]
So, Juanita needs \( \frac{1}{4} \) gallon more of blue paint.