Valleytronic devices based on all-optical ultrafast control are expected to increase the speed of information processing to petahertz and serve a new generation of quantum computers. However, the current difficulty in realizing this vision is the lack of a nondamaging means suitable for ultrafast lasers. We propose a robust scheme to control the valley polarization of monolayer materials, achieved through the quantum interference between 1- and 2-photon transition pathways. The scheme reveals that conventional circularly polarized light is unnecessary for resonantly induced valley polarization and, instead, only a parallel-polarized 2-color field is required. The interference dynamics enables the switch of valley to be manipulated within few femtoseconds without the necessity for extremely strong or single-cycle pulses. The disclosure of this interference scheme enables repetitive operations in valley devices for signal processing at petahertz clock rates without causing material damage. It sheds light on the practical manufacture of high-speed valleytronic devices.