there are several possibilities, assuming that you know the yield stress of your material.
The simplest would be to just change the range for the scale in the post-processor:
Rescaled to a yield stress of 40MPa (red = broken)
You could also use a calculator filter to compute directly the factor of safety (yield stress/ von Mises stress). For values below 1 the part would “break”:
Factor of safety with yield stress 40MPa and a custom range to have a better visual feedback (blue=broken)