Green reads as cool and comparatively muted, with the colour spread fairly evenly. Amber reads as warm and golden, and is usually the more uniform of the two — the impression is of a single consistent warm tone rather than a colour with variation in it. The confusion arises with green eyes carrying strong gold flecking, which in warm light can dominate the overall reading.
There is no threshold for how much gold a green iris may carry before it is called amber, and amber is not a category most classification systems record at all, so the boundary is drawn by the observer.
A warm-biased white balance can turn a gold-flecked green iris convincingly amber, and a cool-biased one can drain amber towards an ordinary light brown.
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.