Both are produced by the same broad light-scattering mechanism in a low-melanin iris, but blue reads as a purer, more saturated blue while grey reads as a more muted, sometimes almost silvery tone.
Many classification systems don't separate grey from blue at all, folding both into one 'blue' category — meaning the boundary between them is partly a matter of which system is used, not a fixed biological line.
Grey eyes are frequently misclassified as blue in photographs because of how camera white balance handles pale, low-saturation irises.
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.