To find the total amount of paint that Serena has mixed, you can simply add the amounts of each color together:
\[ \frac{1}{4} \text{ gallon (blue)} + \frac{1}{4} \text{ gallon (red)} + \frac{1}{4} \text{ gallon (white)} = \frac{1}{4} + \frac{1}{4} + \frac{1}{4} \]
This is the same as:
\[ \frac{3}{4} \text{ gallon} \]
Therefore, Serena has a total of \( \frac{3}{4} \) gallon of paint.