Light brown reads as a single warm brown tone, lighter than ordinary brown but without a distinct second colour family; hazel shows a genuine mix, typically including visible green or grey alongside the brown.
Because the practical difference is 'does a second colour family become visible', and that threshold is a judgement call, sources differ on exactly where light brown ends and hazel begins.
The two are easily confused in photos, especially under warm indoor lighting that can suppress hazel's green component.
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.