Complete heterochromia means the two irises are recognisably different colours — one brown and one blue, for instance. Ordinary asymmetry means the two eyes are the same colour but not identical: one very slightly lighter, or with a little more flecking. Small differences between a person's two eyes are extremely common and are not heterochromia.
There is no agreed threshold at which a difference between the eyes stops being ordinary variation and becomes heterochromia. Sources differ, and mild cases are counted by some and not by others.
This pair is where photographs mislead most often. An angled face lights the two eyes differently, and flash can catch one iris and not the other, producing an apparent difference where none exists.
No photograph or online colour-picker tool can definitively classify which of these categories a specific pair of eyes belongs to — classification is inherently somewhat subjective, and lighting/camera effects change the apparent result.