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