What is the missing term?

The sequence is of the following form:

$\begin{aligned} \blue{a_1} & = \blue{\overbrace 1^{1^2} 0} \\ \blue{a_2} & = \blue{\overbrace 4^{2^2} 2} \\ a_3 & = \overbrace 9^{3^2} 4 \\ a_4 & = \overbrace {16}^{4^2} 6 \\ a_5 & = \overbrace {25}^{5^2} 8 \\ \red{a_6} & = \red{\overbrace {36}^{6^2} 10} \\ a_7 & = \overbrace {49}^{7^2} 12 \end{aligned}$