The difference is where the second colour sits. In central heterochromia the inner part of the iris, around the pupil, is a different colour from the outer part, and the pattern is usually symmetrical — a ring within a ring. In sectoral heterochromia a wedge or segment of the iris is a different colour, running outward from the pupil like a slice, and the boundary is typically a radial edge rather than a circle.
Many irises show some variation in colour between the inner and outer regions, and there is no agreed threshold for how distinct that variation has to be before it is described as heterochromia rather than ordinary iris patterning. Sources also differ on whether central heterochromia should be counted as heterochromia at all.
Pupil size at the moment of the photograph changes how much of the inner iris is visible, so two photographs of the same eye can suggest very different amounts of central colouring. Close-up macro images also exaggerate patterning that is far less noticeable at conversational distance.
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.