Paint Assault

[roadman]

if a randomer in the street threw orange paint on you on what grounds can you punch them in the throat

[Buenchico]

"A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large"
[s.3(1), Criminal Law Act 1967]

The use of force in self defence isn't covered by specific legislation but falls within common law. The principles are laid out on the Crown Prosecution Service website:
https://www.cps.gov.uk/legal-guidance/self-defence-and-prevention-crime

I suspect that you might have great difficulty convincing a court that punching someone in the throat, in the circumstances that you describe, was either 'reasonable in the circumstances' to prevent a crime or 'reasonably necessary' as an act of self-defence. Unless you could, you'd definitely be left up the proverbial creek without the equally proverbial paddle.

[roadman]

i would pull a john prescot and say i thought they were throwing an egg or spraying acid on me or a neurotoxin

[Ringlet]

Depends where you were when the orange paint was thrown.
If orange paint was thrown at you as you were walking down Falls Road, a person might well have grounds for a more serious defence than just punching them in the throat.