Colored double zeta values and modular forms of general level

Gangl, Kaneko, and Zagier gave explicit linear relations among double zeta values of odd indices coming from the period polynomials of modular forms for ${\rm SL}(2,\mathbb{Z})$. In this paper, we generalize their result to the linear relations among colored double zeta values of level $N$ coming from the modular forms for level $N$ congruence subgroups.