A grey iris reads as genuinely neutral — cool, but without a colour you would name. Blue-grey reads as neutral with a blue lean that becomes obvious in daylight and recedes indoors. The two are close enough that many people use the words interchangeably for the same eyes.
Blue-grey is not a category classification systems record, so the question is really how much blue a grey iris may carry before an observer names it differently — a judgement, not a measurement.
This is the pair most easily moved by automatic white balance: a single processing decision can shift the recorded colour from one side to the other.
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.