Using a three-dimensional classical ensemble model, we investigate ultrafast dynamics in nonsequential double-ionization (NSDI) of Ar atom by co-rotating two-color circularly polarized laser fields with the frequency ratio varying between 2 and 5. Numerical results indicate that the NSDI probability gradually increases with the frequency ratio between the two components increasing. The probability for the frequency ratio 5 is one order of magnitude higher than for the frequency ratio 2. Back analysis of NSDI trajectories shows that recollision occurs mainly via a short looping trajectory. With the frequency ratio increasing, the traveling time of the free electron shortens. Furthermore, the relative contribution of recollision-induced excitation with subsequent field ionization mechanism in NSDI gradually increases as the frequency ratio increases. It is attributed to smaller recollision energy and larger recollision distance for larger frequency ratio.