  We study color entanglement effect for T-odd cases in collinear twist-3
factorization. As an example, we compute the transverse single spin asymmetry
for direct photon production in pp collisions in pure collinear twist-3
approach. By analyzing the gauge link structure of the collinear gluon
distribution on unpolarized target side, we demonstrate how color entanglement
effect arises in the presence of the additional gluon attachment from polarized
projectile. The result is consistent with that obtained from a hybrid approach
calculation.
