Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek urgent or emergency care for:
Sudden loss or marked reduction of vision
Severe eye pain, especially with redness, nausea, or headache
A chemical splash or penetrating eye injury
A colour change after significant eye or head trauma
A new drooping eyelid or unequal pupils with weakness, confusion, or severe headache
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Contact an eye care service promptly if a colour change occurs with ongoing redness, mild eye pain, sensitivity to light, new floaters, or gradual visual change.
These symptoms can occur with inflammation or other treatable eye conditions.
3
Does any of this apply?
Book an appointment
Arrange a routine eye examination if heterochromia is newly noticed, has not been assessed before, or appears to be changing.
A child with a persistent colour difference should have the eyes checked, particularly if there is hearing loss, a drooping eyelid, unequal pupils, or unusual hair or skin pigmentation.
The article also says
A stable colour difference without pain or vision change is not usually an emergency.
Every sign above is quoted from the sofidex article on Heterochromia. If you are not sure, err upward — it is always reasonable to be checked.