  Color-kinematics (CK) duality is a remarkable symmetry of gluon amplitudes
that is the key to the double copy which links gauge theory and gravity
amplitudes. Here we show that the complete Yang-Mills action itself, including
its gauge-fixing and ghost sectors required for quantization, can be recast to
manifest CK duality using a series of field redefinitions and gauge choices.
Crucially, the resulting loop-level integrands are automatically CK-dual, up to
potential Jacobian counterterms required for unitarity. While these
counterterms may break CK duality, they exist, are unique and, since the
tree-level is unaffected, may be deduced from the action or the integrands.
Consequently, CK duality is a symmetry of the action like any other symmetry,
and it is anomalous in a controlled and mostly harmless sense. Our results
apply to any theory with CK-dual tree-level amplitudes. We also show that two
CK duality-manifesting parent actions may be factorized and fused into a
consistent quantizable offspring, with the double copy as the prime example.
This provides a direct proof of the double copy to all loop orders.
